check skinny's chart

as you slide back or move seat back you 'push' more of the stroke cycle, using the extensors.
move the saddle forward and you use the flexors more (in proportion)
the gluts provide an intermediary and critical role as well as 'starting' the stroke as hip extensors, so they remain primary regardless of the position.
climbing in the saddle usually invvolves some movement rearward, meaning more pushing, more extensor engagement. spinning in an in-saddle sprint usually means you go forward on the rivet, more flexors, and if the saddle height isn;t right the hammies will be complainin big time.
this assumes a 'normal' road position.
The more forward position of the modern TT is such that, even though forward, it still allows a strong engagement of the extensors (quad muscle group).
flat, toe down or 'ankling' pedal style are all valid and are/should be adjusted for in 'position'.
dropping the heel at the top of the stroke isn't an advantage unless you are really far back. In a normal KOPS position a dropped heel at the top delays the engagment of the gluts - not the best thing.
ankling does have an advantage that it reduces and slows the required movements of the rest of the leg, which carries much more mass. Slower and less movement means less muscle opposition and the possibility of greater rpms. But doing that means the lower leg muscles - flexors and extensors take a bigger hit of the effort.
Current thinking is for some ankle flex, but much less than what one saw in the premier pedalers of the 50s, 60 & 70s. Trackies tend to be more supple in stroke than roadies, since the fixed gear requires the engine to do all the 'transmission' work.