Requirements:

Javelin/Stinger/RPG-7
Frag/Semtex Grenade

Press Y so you have your Javelin/StingerRPG-7 wielded.

Press and hold RB so you are holding your grenade.

Press Y while still holding RB.

When your Launcher is wielded again, et go of Y and when someone kills you, the launcher explodes where you were standing, which gives you a comeback most of the time (+50 xp).