Originally Posted by Bob Dopolina

Being behind the gear usual means you are slid back on the saddle, heel down, with a lower cadence and you are driving hard focusing on the driving across the top of the pedal stroke. Like seated climbing.

Being on top of the gear is the opposite. You slide more to the nose of the saddle, at a higher cadence and some raise their heel slightly. Being on top of the gear means you can spin the gear up without having to stand.

Think of an attack where you are out of the saddle. That is the initial jump. Once you sit back down you should be on top of the gear and driving along at a good tempo.

Make sense?