Instructions
Sign on to the co-op mode. This process requires a partner.
Access the Help Room and buy a flame thrower or a browning gun.
Go to the right corner outside of the Help Room. Have your partner lay down right in front of the drawers next to the corner. Jump on top of your partner while he is lying down, and then crouch.
Have your partner crouch. The glitch activates when your bodies merge together. As long as your bodies are together, the zombies cannot inflict any damage on either of you. Neither player should stand up or walk out of this position, or else your bodies will separate and you will no longer be invincible.