Underdrive Pulleys

Hey, me again...I was looking on ebay and reading in some articles about underdrive pulleys and i have a few questions:

1) How do you install? Am i gonna have to pull the engine because the pulleys are to the right side and theres no way i can really work on it, unless its the crank pulley(pull fl tire off?)

2) How much horsepower does it really add and how? (i've read it has to do with weight, does that allow it to spin faster or something?)

Thanks gurus.