Well on the other forum I used to visit, this was the number 1 topic.
What you do is draw the persons username that posted last, so someone will draw me, then someone will draw the guy that drew me.
So someone will draw what they think of when they see my username.
I remember on the last forum I went to, there was a guy called 'Zombiekillingninja' and this one guy did a really awesome drawing in photoshop of a zombie with a throwing star stuck in his eye. I'll look for that picture now.