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This is a technique where the user focuses chakra into their hands and feet using precise chakra control and releases it with pinpoint timing, which greatly enhances their strength. Tsunade created the concept of this technique to intimidate enemies and keep them at bay while she healed injured ninja.
The training for this technique is quite arduous, as Tsunade repeatedly came at Sakura with full power to help her learn. Despite this, the outcome of this training is incredible, giving the user enough strength to easily make gigantic craters or kill opponents with single strikes.
While some may see it as "Superhuman Strength", this technique is actually an application of medical ninjutsu that demands excellent concentration and chakra control.
It should be noted that unlike Sakura, Tsunade herself already possesses incredible raw strength, capable of crushing stones in a single hand with nothing but pure muscle power. This strength is later increased to monstrous levels, when Tsunade uses this technique, allowing her to perform feats like lifting and swinging Gamabunta's sword high in the air.