This cheat can be done in any corner of the game. But where I mostly do it is the corner by the boxes where the mystery box is (in Nacht der Unoten).

You have a friend or you stand in the corner and jump up and down (keep jumping until the person has performed next step).

The other or you, go prone, and crawl under the person while they are jumping.

When you are completely under the jumper or your partner is under you. Have your teamate stop jumping and you stand up.

You both should be inside each other (you should see through them or eyes etc).

Now you are invunerable to zombies. When your want to go to the box have you and your partner gently move to the box.

When both are in reach of the box (it should show "Press X to use box") you both are set for ammo and are invulnerable.

Note: no grenades or flamethrower gernades hit Zombies and kill you and flamethrower can burn your partner.

Now, go have fun.