What are underdrive pulleys?

This kid with a mustang at my school was talking about his car today and said he had done sorts of stuff to it. Its a v-6 automatic and he said he could beat GT's in it. So i listened to him for a minute. Hes got all sorts of stuff, a chip, cold air intake, but he was also talking about underdrive pulleys. what are underdrive pulleys??
ive never heard of them before, but if this kid claims to be beating GT's with them, then they sure as hell must boost the horsepower by a good amount.
any help is appreciated. Thanks

They are pulleys for the accessories and crank that basically slow down the drive belt driven accessories. The idea being to put less drag on the motor. Depending on the engine they have varying success. From my experience they work pretty well on 302 Mustangs (10-15 rwhp) with no side effects, okay on the 4.6 Mustangs (5 rwhp with some charging effects; ie with the head lights, heater, and stereo on, there is enough power loss to dim the lights), and generally don't do shit for Hondas power wise and can actually cause problems since none of the manufactors for Honda under drive pulleys seem to see it fit to manufactor a descent harmonic dampener for the crank pulley.

I have no idea about the V6 Mustang parts. If he is comparing a 99+ V6 with 96-98 GTs then it is entirely possible that a few modifications would put him close or faster. The 96-98 GTs were dogs in the late 1970s caliber, ie very piss poor bad. The V6 has a weight advantage also...

I'd find it hard to believe that hose mods would be having a V-6 beating a GT IMO

Yah, i had trouble believing this kid, and i still think hes BSing about the fact that he can beat GT's. I was just wondering if underdrive pulleys would actually give an amazing amount of horse power, which they obviosly dont. thanks for the info.

my friend beat a stock V6 mustang with his CRX that has Intake, Exhaust and a milled head.