Super TIME Force. Dying forces a Time Out (the rewind mechanic), meaning you can go back in time to an arbitrary point before you died, and spawn a replacement version of the same or a different character. If you can save your original character (by blocking the shot that killed them, or killing the enemy that killed them before they can kill the original, for example), the point where that character died in the original timeline is preserved as a powerup that you can pick up, giving you an extra hitpoint (it's usually one-hit-kill), and if it's a different character, combining your alt-fires together.
It's kinda hard to wrap your head around at first, but amazing once you do.