What's a under drive pulley?

[jpblue1000]

in my circles we fit varied ratio pullies 'underdrive' on engines who spin faster than the manufacturer intended. this in turn reduces the speed of the water pump in particular to a level more akin to what the manufacturer designed. In turn this reduces water cavitation at the impeller maintaining better cooling efficiency. the water pump speed has been returned to 'normal' speeds. secondary the alternator does not like the additional 2-3+k extra revs, reducing its speed lengthens its service life. we have not experienced any real gain in engine performnce maybe 1%. we dont run a/c or p/s but sometimes add external oil pumps.
JP