If you flop the under full, that is, say you have 44 in the hole and the flop comes comes 4-10-10, bet out, you want to catch somebody with three tens to come along for the ride. If you flop the over full, then you slowplay it since you have the deck crippled you want the others to get help on the next cards or bluff at the pot.
Edit...Jesus, I just realized this is almost verbatim what Doyle says in SS. I hope I'm not sued for plagarism.