Read up. Basically he controls 5 accounts at the same time using 1 set of commands. He either does this by having 5 keyboards attached to 5 pc's (like the DAOC guy) or by using a program that uses 1 keyboard to control all 5 accounts and make them do the same thing. Then he can just scroll his mouse across 5 monitors (or, again alternatively he can switch screens on the same monitor).
Eg, he is 5 boxing 5 mages. He casts a fireball, and all 5 mages cast a fireball at exactly the same time. He can rig up basic commands like Xzin who has somehow managed to macro making his 9 other characters spread out, although lord knows how.
It'd be interesting to see a pic of your setup.