Time Bomb Seal

In a room lit with blue-flamed candles, Tsunade places a seal in Kakashi's inner forearm. The seal is set to activate no matter what happens with its wearer. The seal was set to activate Kakashi's Mangekyō Sharingan during Hiruko's ritual to assimilate his body. Sai mentions that this is used as a last resort technique by the ANBU.