Somersault Kick is one of Charlie and Guile's special moves where they perform a backflip kick which releases an air-slicing projectile wave diagonally-upward. Guile's version is done with an actual backflip kick while Charlie's version is done in backwards fashion where he performs a front-flip-kick while facing away from his foe.
An unrelated move returns in Marvel vs. Capcom 3: Fate of Two Worlds and its Ultimate update as a special move for Jill, but it cannot be executed without entering Feral Crouch, as this is a special stance heavy attack.