The best description that I could find is that a power change doesn't "float" up to the plate ala Tom Glavine. So basically it has a good amount of downward movement.

 

They've been compared ad nauseum, but it sounds like Floyd's changeup. The word power seems to make it sound really cool and good, but it's probably a tough pitch to control because it'd be easy to leave up to hitters, and a pitch that drops into a hitter's wheelhouse at 83 MPH will go a long ways, just as a straight fastball at 96 in the same place goes a long ways.

After a quick search, some pitchers that have been described to have power changes are Hamels, J.Santana, Okajima, J.Francis, R.Ramirez. Lots of words like "drop" and "dive."