Underdrive pulleys are usually larger than stock, which spins the accessories slower, which saps less power from the engine. They're also usually lighter than stock, which also makes them easier to spin than the stock pullies. The downside, at least for the alternator pulley, is that Civics usually have pretty weak alternators to begin with, and if you start slowing down the alternator pulley, it may not put out enough current to feed your electrical system if you have a lot of stereo equipment, etc.