underdrive pulleys

i have been lookin into pulleys lately and jegs sells a set for my truck for 130.00 bucks http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=2116&prmenbr=361 wut im askin is how does pulleys free up the 10-15 horsepower these are saying? are the pulleys normally bigger or smaller? and will they hurt me if im in overdrive at a real low rpm? wut are goods and bads about underdrive pulleys?