On the Gridlock map as you are running up to the middle were the sniper is there will be 3 bits of cover to your right.
Get your lancer out and stand at the top of the stairs and look at the far part of the cover, press A and B at the same time.
You will attach to the cover but you will be floating, crouch down and then switch to the low wall close to where you're standing.
You'll notice that your guy hasnt switched cover which is supposed to happen.
After that look in the opposite direction and fire your lancer.
After you fire the lancer you will be able to move.
On your screen it will look like you are standing up but with one noticable difference - you are twice as fast, whereas other players will see your crouching.
While in this glitch you can still shoot although you are crouchrunning.