Hundred Strength Seal is a fuinjutsu that allows the user to accumulate a huge amount of chakra in one point of the body, usually on the forehead, over a long period of time. This forms a diamond-shaped mark. The technique requires extremely delicate chakra control, available only to a few. Once formed, the seal enhances the already high chakra control, allowing techniques to be performed without loss of energy. When released, the seal spreads across the face or body, releasing the accumulated chakra and multiplying the power of all the user's techniques many times over. The mark will appear on the forehead again when the user accumulates enough chakra in that place once more.
The seal is symbiotically linked to the summoning of Katsuyu. This allows for mass healing of many people or maintaining contact with the slug over a long distance. The power of Katsuyu's techniques directly depends on the chakra reserves in the seal. Tsunade and Sakura Haruno, when releasing the seal, combine its power with the Divine Regeneration Technique, gaining access to the Ninja Art Divine Regeneration - Hundred Strength Technique.
It took Sakura three years of continuous chakra accumulation to form the seal. Shizune, Tsunade's first apprentice, was never able to achieve this due to an insufficient level of chakra control. Tsunade also uses part of the chakra from the seal to maintain her youthful appearance.
