You need to be on co-op or Xbox Live to do this glitch.

First you or your partner unlocks the help room then you should by a good gun (flameflower or browning).

Then go to the corner beside the help room. The one right to the room.

Then one of you lies down about a centimeter from the drawers and the other goes on top of his chest when he's still lying down.

Then you tell him to stand. You will know you are in the cheat because one of you will be inside the other persons body (make sure not to move or the glitch will be ruined).

Zombies will now come towards you and you will survive.