I've got a copy of Phil Burt's book
Bike Fit which I've found very helpful. The guy below appears in a bunch of photos demonstrating the extremes and the ideal according to Phil. When I'm riding and setting up my bikes, I try to imagine myself taking on the form of the posture on the left. That said, that is obviously not what pro riders look like so I imagine that they are both:
a) Much more flexible and core strong than recreational riders and;
b) Possibly making some back health compromises in the name of kicking ass.
I've only been riding with any seriousness for about three years now. In that time, my preferred effective top tube dimension has gone from about 57 cm to about 61 cm as a result of the changes to my conditioning. It feels as though it comes down to being physically capable of getting a bit "long and low" without having to tilt your pelvis -- or your seat -- too far forward.
My experience with trying to create my own weightless hands scenario has been that these things tend to promote it:
1) Butt further back.
2) Longer and lower so long as my pelvis isn't tilted greatly forward.
3) More pedal force to balance the shift in weight centroid produced by #2.
