Be sure to set your platforms at or below centerline, abrade well, experience will tell you at what angle to hold the piece and what angle to strike it, then just rare back and knock the crap out of it
. Easier said than done I know. Accuracy while doing all that is of the utmost. Hit either side of platform and (snap!) aim too steep (snap or step!) get all wimpy with your swing(step stack!).
Hit one of those knap ins up there, get with somebody. Then practice, practice, practice.