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.