What exactly is a Under Drive Pulley?

A lot of people tell me to put this in my car for my next mod after intake and exhaust, is it worth the $100-300, and do these work with automatic transmissions or manual, and where should i look into getting one for a NA 3000gt EBAY?

ebay, 3sx.com, dynamicracing.com, ipsmotorsports.com... I like mine..

i personally recommend the 3sx lightweight pulley. The underdrive pulley is a smaller pulley in general, and underdrives all your accessories (stereo, lights, a/c, etc) so that they don't have as much power to operate off of. The 3sx lightweight pulley is the same as the stock size pulley, just much lighter, so you free up power due to less rotational mess but you don't underdrive your accessories. I think its down to arround $100 or so, so its definitely worth the money, especially if your N/A. Both types of transmissions have the same engine so whether you have a manual or automatic doesn't make a difference.

they are actually quite a big debate topic. Theres been many threads, so im sure if you search around a little, youll find TONS of info.