Mark, under drive pulleys spin the alternator, AC compressor, power steering pump, and water pump slower, thus supposedly freeing up some more HP.
( smaller crank pulley, larger alternator and PS pump pulleys, same belt )
I don't believe anyone has ever posted any dyno runs confirming this, and exactly how may HP you get for the $$$$$.
I messaged with latemodel21 back early this year about making a few sets,
I will defer to his expertise as he advised me to not waste my money.