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From Tekkenpedia

The Rage System was a system that was first introduced in Tekken 6, though a similar system, Netsu Power, had been implemented already in Tekken Tag Tournament. The Rage System activates when a characters health drops below a set point, whereafter the character enters "Rage" and has increased attack power. The set point was initially set to 10%, but after complaints of it being too high, Namco created a patch where the set point was decreased to only 5%. During Rage, a characters hands and feet will have an afterglow, which reportedly is default red in all characters, but can be customized into other colors.