Yin Seal

This is a seal unique to Tsunade, created by storing chakra over a period of time in her forehead. At a time, she is able to release the stored chakra, thus greatly increasing her chakra flow. Sakura Haruno was able to create a stronger variation of this technique, as she states she is able to use her seal's power more liberally than Tsunade.

Sakura stated, that with the application of this seal, Tsunade is able to maintain her youthful appearance.

Trivia

 * It is unknown if the mark on Mito Uzumaki's forehead is related to the Yin Seal.