Instructions
Choose your custom class. Pick javelin as your secondary weapon and semtex as your equipment. Your primary weapon and special grenade does not matter.
Press the "triangle" button on the PS3 controller to switch to the javelin rocket launcher.
Press and hold the "R2" button on the controller. This is the designated button for your semtex grenade.
Press the "triangle" button again to switch back to the javelin. Let go of the "R2" button when the javelin rocket launcher is back in your hands. You are now ready to explode. You can knife, sprint and jump. Anything else you do -- such as firing your weapon -- cancels the glitch and the process has to be repeated.