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The Strength of a Hundred (百豪の印, Byakugō no In) is a sealing technique introduced in Naruto chapter 153, borne by Tsunade. This Seal takes the shape of a diamond and is positioned in the center of her forehead.

In chapter 169, the manga reveals the utility of this unique seal. When Tsunade releases this seal, it shines and a series of lines start covering her face.

Tsunade calls this action Creation Rebirth (創造再生, Sōzō Saisei ), also known as Mitotic Regeneration.

She releases this technique with a hand seal Yin Seal: Release (陰封印・解, Infūin: Kai) or Reserve Seal: Release.

The Creation Rebirth jutsu heals Tsunade’s most fatal injuries without Medical Ninjutsu.

Once used, the seal disappears.

This jutsu has regenerative properties.

Tsunade, the greatest medic in the world, explains that she developed this technique by storing chakra on her forehead and releasing it all at once to accelerate the division of cells and accelerate the building process. 

The Creation Rebirth serves, in part, as a substitution of the Mystical Palm Technique, also known as Medical Ninjutsu, which normally requires hand seals and wastes large quantities of chakra. 

The proficiency required to master this technique has made it known as “the absolute pinnacle of Medical Ninjutsu”.

This regenerative technique was created by Tsunade herself, as people like Orochimaru and Madara Uchiha have never heard of this technique before.

To elaborate this technique, Tsunade probably took inspiration from her grandfather, Hashirama Senju, who boasted large quantities of chakra and did not need to form seals to heal his body.

Hashirama used a similar regeneration ability according to Madara Uchiha and the fourth databook.

Fourth Databook, page 165.

This is the ultimate reason why the Creation Rebirth cannot work without the chakra reserves of the Strength of a Hundred Seal (百豪の印, Byakugō no In).

To perform Medical Ninjutsu, the user requires high reserves of chakra to accelerate the natural healing processes of the body.

Fourth Databook, page 279.

However, this jutsu presents a disadvantage to the user: it shortens the lifespan.

According to Shizune, Tsunade's disciple, the number of times the cells can be divided is limited and speeding this process shortens the lifespan of the person.

Forbidden jutsus, also known as (禁術, Kinjutsu), are techniques not recommended among Shinobi because of the life threatening conditions they have.

First Databook, page 159. 

The Creation Rebirth (創造再生, Sōzō Saisei) is a S-rank forbidden regeneration technique classified as Medical Ninjutsu, which is, as aforementioned, a healing technique that wastes large quantities of chakra.

The Creation Rebirth covers all the qualities of a forbidden technique: it shortens the lifespan, therefore it poses a danger to the users life; and it goes against the laws of nature, as it is impossible to regenerate limbs and organs from scratch or accelerate the division of cells at such speed rate.

Second Databook, page 242.

Tsunade uses the large reserves of chakra stored inside her Seal to use this regeneration technique.

So, in essence, the Strength of a Hundred and the Creation Rebirth are linked by the chakra inside the Seal. The Creation Rebirth needs the chakra inside it in order to be able to replicate the cell division and allow regeneration because these two actions require vasts amounts of chakra.

To explain this a bit further, the Creation Rebirth (創造再生, Sōzō Saisei) is a technique, created by Tsunade, that is “stored inside” a sealing jutsu called the Strength of a Hundred Seal (百豪の印, Byakugō no In). The jutsu cannot be used without the Seal unless the user possesses high reserves of chakra.

Thise Seal’s only purpose is to store quantities of chakra inside to be used by the person bearing this Seal. When released, this Seal amplifies the abilities, the performance and the power of the jutsus as the chakra extends around the body.

First Databook, page 158.

The amount of chakra that can be stored inside the Seal seems to be limitless and there is no confirmation about the capacity of the Strength of a Hundred.

Apparently, by the time of the War and after Sakura had used its reserve several times, the Seal has more chakra inside than a clone of Six Paths Kurama Mode.

It is unknown how much time it took Tsunde to reunite the amount of chakra necessary to manifest the Seal.

According to Shizune, the Strength of a Hundred Seal is a jutsu that requires extremely precise and delicate control to amass chakra in the same place for 3 years.

It would not be wrong to address this jutsu as "the pinnacle of chakra control" as well.

This is also explored in the anime. I am not an author that includes the work of Studio Pierrot in my works, but the explanation given here matches the manga and it serves as an explanation.

Sakura spent 3 years continuously controlling her chakra to store it in a reserve that would later become the Strength of a Hundred Seal.

Episode 408.

Once Sakura awakened this Seal in chapter 632, her strength was vastly increased.

She said that once she achieved her full power, she would be able to unleash it.

However, she did not release the seal at that exact moment.

There is a possibility that the mere awakening of the seal augments the user’s abilities to some extent.

In my opinion, this is not like that. The anime might explain the reason why Sakura was able to release her truly massively destructive power.

It is possible that, up until that moment, the manga had never shown Sakura Haruno’s full strength.

The anime mentions that her chakra reserves had become low due to the effect and attention Sakura is directing to the completion of the Seal. This led her to faint or to be prohibited to perform Medical Ninjutsu or go into battle by Tsunade.

To counter this impediment, Sakura created a second reserve, so she would direct chakra to the Seal and dedicated another smaller reserve to use in battle and healing.

Contrary to popular belief, the Strength of a Hundred Seal was not created by Tsunade.

According to the Boruto manga chapter 36, the Strength of a Hundred Seal exists since the age of the Sage of Six Paths.

Mito Uzumaki bore a mark on her forehead similar to the Strength of a Hundred Seal.

However, it is unconfirmed if this mark is the seal or not. If it were the case, Tsunade must have probably taken inspiration from her grandmother to store chakra.

When used together, the Strength of a Hundred and the Creation Rebirth are known as Ninja Art: Creation Rebirth - Strength of a Hundred Technique (忍法創造再生 百豪の術, Ninpō Sōzō Saisei — Byakugō no Jutsu) and is known as the ultimate regeneration technique.

Altogether, the Ninja Art: Creation Rebirth - Strength of a Hundred Technique is an S-rank supplementary ninjutsu, classified as well as Kinjutsu and Medical Ninjutsu.

Fourth Databook, page 279.

The medic that completes this jutsu is able to fight in the battlefront according to Tsunade’s medic rules.

As long as this jutsu is activated, the user will not die.

In order to release this powerful technique, Tsunade uses Yin Seal: Release (陰封印・解, Infūin: Kai) or Reserve Seal: Release.

Second databook, page 242.

Basically, Yin Seal: Release is the hand sign of the tiger used in order to release the seal and the Creation Rebirth. It is not another name used to denote the Seal or the jutsu.

Tsunade uses this hand seal again in chapter 562 to access the Creation Rebirth jutsu.

However, there have been changes since its debut in chapter 169. After using this jutsu once, the seal does not disappear from her forehead.

This hand seal is not strictly necessary to release the Strength of a Hundred jutsu. Tsunade did not use hand seals when she fought Madara.

Sakura Haruno mastered this technique and made no use of hand seals to release its power either (the chakra stored inside the Strength of a Hundred Seal can be shared with others as a Chakra Transfer Technique as well).

The name Yin does not make Medical Ninjutsu a Yin Release ninjutsu type.

In fact, Medical Ninjutsu leans more towards Yang Release, as it is a technique that deals with the body and regenerates physical energy.

Every single medic-ninja known in the series has a Yang affinity. The most important known medic-ninjas in the series have this chakra nature except for Shizune or C, but this is because their chakra affinities have not been shown in the fourth databook.

― Tsunade: Yin

― Sakura: Yin and Yang

― Shizune: not shown

― Rin: Yang

― Kabuto: Yin and Yang

― Ino: Yin and Yang

― C: not shown

Elemental chakra is the only type that, once transformed into ninjutsu, does not require Yin or Yang Release, as these affinities exist outside of the five elements.

Many techniques, especially Hiden, are linked to these 2 chakra types, for example Nara Clan and Akimichi Clan techniques are linked to Yin and Yang Release respectively. It is possible that the Yamanaka Clan techniques are linked to Yin Release and that Medical Ninjutsu is linked to Yang Release.

If logic is applied, Medical Ninjutsu responds to the physical energy of Yang Release. Tsunade does not possess Yin Release, but this factor does not matter as it is a hand seal the requirement to release the power.

Still, there is no confirmation about the nature type of Medical Ninjutsu or the nature type of Seal releases.

In addition to the Creation Rebirth, the Strength of a Hundred Seal is also linked to Katsuyu.

Fourth Databook, page 79.

Katsuyu is a giant slug that has regenerative properties and can heal the most fatal of wounds.

It originates from the Shikkotsu Forest and its true size is completely unknown. Working together, Tsunade says that herself and Sakura can summon at least one-tenth of its body.

It is probably the most powerful summon in the series as it is practically indestructible.

Katsuyu was able to protect people and survive Pain’s destructive Shinra Tensei and Naruto’s tailed transformation.

Its mollusk body can divide itself into multiple tiny fragments to avoid attacks and also turn itself into a liquid to share its properties with people standing on the floor.

The ability to divide its body in tiny fractions functions like Shadow Clones.

Fourth databook, page 238.

This is explained by Katsuyu itself in chapter 635.

The ability to use the healing network depends on the Seal. This would also explain why Katsuyu does not have a summoning limit time like the rest of summons.

When the chakra of the Seal has been completely exhausted, the mark disappears. Although the ANBU says that Tsunade has shared the Creation Rebirth instead of healing chakra, I am more inclined towards a mistranslation. 

Tsunade must have shared the chakra stored inside the Strength of a Hundred but not the regeneration jutsu, as it is a very powerful technique to use with every single inhabitant of Konoha all at once. 

This is an assumption of mine.

As a side effect, Tsunade returns to her wrinkly and older appeareance. Tsunade uses the chakra inside the Seal to appear younger, so the moment this seal disappears, the effects are reverted. 

However, it is curious that without the Seal, Tsunade looks older than Jiraiya despite being the same age and also being an Uzumaki, a clan known to live long lives.

It is possible that her body might have accustomed to the Seal and without it, his body feels consumed. There is also the possibility that the extended use of the Creation Rebirth had taken a toll on Tsunade's health, as it is a technique that consumes the body. 

Tsunade uses the chakra inside the Strength of a Hundred Seal to maintain the effects of a special Transformation Jutsu and keep a youthful appearance. This is something she does on purpose, it is not something that comes with having the Seal.

Sakura, on the other hand, boosts the Seal by not wasting any chakra to appear younger.

This could be the reason why Sakura may have more chakra reserves stored than Tsunade, but this is only a speculation.

Sakura does not use the chakra to look younger in Naruto Gaiden.

Katsuyu is not only useful as a support summon for healing but also as a defense and evasion. She can use her division to avoid attacks and also spill acid.

Second databook, page 241.

The ability to summon Katsuyu is linked to the Strength of a Hundred Seal itself.

Sakura was never shown summoning Katsuyu until she awakened the Seal and Tsunade is shown bearing it in a flashback from the Third Hokage in chapter 634.

Shizune has never been seen calling Katsuyu.