Overdrive pulleys increase the rotational speed in relation to the engine. Basically your idle RPM should make the same current as an RPM 30% higher would make with the stock pulley.

This comes at a cost too though. If it spins faster, it ages faster... And yes if you rev the engine to a point that originally would have met the top rpm for the alternator, with the pulley it will exceed that point and probably just age the coil even faster.