Instructions
Select any mode of game play and choose Sasuke as your character when prompted at the Character Select screen.
Allow Sasuke to take the requisite amount of damage. Examine Sasuke's health bar to see a small marker. All the characters in the game that have wakening Mode," such as Sasuke, Naruto, Jiraiya, Kakashi and others, have this marker, but it is in different places for each one.
Wait until Sasuke's health drops below his Awakening Mode marker. When this happens, you will see a flash of flames move across his life bar. This happens very quickly, so the timing can be difficult. Press and hold the Chakra charge button, Y or Triangle, depending on which console you are using, as soon as you see the flash move across your health bar.
Hold the Chakra charge button beyond the point when your Chakra meter is full. Continue holding it and you will eventually see a transformation sequence as Sasuke unleashes his Mangekyou Sharingan.