The Sideways Somersault (often short called Side Flip) is one of the Jumping moves in the classic Tomb Raider Games and Tomb Raider The Angel of Darkness. It is performed by pressing sideways and jump while standing still.
Since Tomb Raider Legend a sideways somersault can only be performed when in combat mode.
There is no point in the games where you have to perform a sideways somersault, it can always be replaced with another move, however it can make life easier when jumping through a Stargate, jumping up blocks (e.g. when in the elevator shaft in Tomb Raider Chronicles) or evading enemies.