When you are low on the bike, there is a steep angle between your thighs and your upper body and at the bottom of the pedal stroke, your hamstrings are quite stretched. The more relaxed and 'within it's limits' your body is, while in that position - the more power you put through the pedal stroke.

This concept is central to the Serrota fitting system, and this is why they sell bikes that are made for real roadies and racers who care about comfort and power transfer, not poseurs who want to ride a bike that looks like Jens Voigts!