You need to have downloaded the new map pack and when you do the glitch you need to be on the side with the kar98k is.

Go to where the wall with the grenades on.

There are two ways to do this glitch.

To do the glitch you can sprint at the wall, jump and crouch when you hit the wall, or you can move and touch the wall (in prone) and crouch after you touch the wall.

If done correctly you can't be killed.