Whether a DMW is legal depends on the tournament. Most majors (NPPL, Mil, PSP) allow it. Basically, it's what Weedave said - act like you're out, mosey towards the deadbox, wait for the other team to get near, then light 'em up. Unless it's down to 1 or 2 players on the other team, expect to get lit up in return.
Talking to yourself, cursing your gun/teammates/refs and messsing with your gun can help you pull off the DMW.