underdrive pulleys???

i know this is prob a stupid question but what is an underdrive pulley?? i saw on 3sx that its a common upgrade for n/a 3000gt's...ive heard some people say it hurts the engine and some say their wrong so i need some opinions...also what do they do and how much do they cost?? thanks

This should cover it all:
http://www.pacarsearch.com/stealth/udp.htm