A BxR is a button succession in Halo 2, in which you can use any human weapon and select covenant weapons to instantly kill an opponent. With default controls, you press the melee button, "B" by default, to beat-down, and as soon as your attack meets its target, hit X and R as fast as you can keeping your target reticule on their face. This will drop their shield, and kill them if done correctly. As said before, this can be done with a pistol, a sniper rifle, a battle rifle, and carbine.
HaloPlayer1: -BxRs- HaloPlayer2
HaloPlayer2: WTF, way to cheat.
HaloPlayer1: Not cheating. Owning. Get Raped!