From Halopedia, the Halo wiki
The Suicide Shot is a button combo that can be used to commit suicide without losing a point. It is useful for getting out of a bad spawn.
When a player rapidly presses the button sequence R + R + Y, and moves forward, a series of events happens that results in their death. The player fires a single shot before switching weapons as they fire the second shot. Both shots come from the position the player was in when they started the button combo, so if the player moves forward, the second shot will actually be fired from behind them, and hit them in the back of the head, killing them instantly. The kill, however, will be credited to the The Guardians, and the player will not lose a point. MLG prohibits this button combo.