From Halopedia, the Halo wiki
Instant Death is a button combo present in Halo: Combat Evolved and Halo 2. The button combo can instantly kill a target when used with a Battle Rifle or Sniper Rifle, though it is widely considered the hardest button combo to pull off. The combo is executed by aiming at a target's face (the front of their head) and rapidly pressing R+R+Y+B or B+X+R.
When a player executes the button combo, a sequence of events occurs that can cause an instant kill. They fire a single shot, switch weapons, melee, and fire a second shot, killing the foe. The melee (B) is to be performed when the secondary weapon appears on-screen. R+R+Y+B is superior to B+X+R because it always does damage (because of the first shot) and it does not require a partially-empty magazine (as no reload is performed).
Note that if a player runs forward as they perform the combo, then their second shot will hit them in the back of their head, killing them rather than their opponent.