When the mosh pit splits in half and runs at each other at a certain point in the music. A good song to use is Duality by Slipknot.
Usually followed by much moshing
Joining a wall of death is very scary, but one tip, is to take a deep breath before running, and look at the wall of people in front of you and decide on who you're going to run at.
It's a huge adrenaline rush.
Harry: Dude, wanna go join the wall of death?
Sally: Nah, I got a bruised eye moshing with those huge guys earlier. I'm out for the rest of the night.
Should be avoided at all costs. The area between genitals and ass hole - usually only explored up close when performing oral sex.
'I went down on Sheila the other day and her wall of death smelt like frog spawn'