[QUOTE=tomato coupe;22497513]Below is a level foot at the bottom of the stroke. I don't see how a level foot can be considered a "well dropped" heel.[QUOTE]
That's a slightly cherry-picked photo. The pedal is well forward of BDC and the heel is already below the toe. As the pedal comes back, the angle between the bottom of the foot and the tibia will further decrease. The objective of the heel drop is to relax the calf as much as possible, consistent with being able to pull the heel cup straight back at the bottom of the stroke, no calf muscle involved. Here's some serious pro heel-dropping with bottom of foot more or less level, this time due to the butt being well forward of the usual road position:
Plus it's on the flat with a high inertial load, so one would expect to see more hammering on the downstroke. Note that during the pull-back phase, the bottom of the foot is square off the tibia, just like it would be if one were in a more upright climbing position. The TT position means that BDC happens maybe 20° later w/r to the horizontal.
As for the why this works for at least some of us: At high speed on the flat there's a high crank inertial load. There are many research papers which show that as crank inertial load increases so does cadence in trained cyclists. As cadence increases, power production is almost entirely during the downstroke. But there's a however. Climbing, crank inertial load decreases by as much as 90% and cadence also decreases, although power production actually increases.
Power is a function of cadence and pedal force. So if power is going to increase and cadence decrease and pedal force continue to be only during the downstroke, as some folks obviously believe, pedal force is going to have to increase a lot. Not going to happen on a long climb. Instead, what happens is the dwell time of the pedal force increases. That pedal force is probably less when our TT champion is on a long climb than when they TT, climbs taking so much more time, so the dwell time has to increase by a lot. Therefore they start pushing forward at 11:00 and pull back until maybe 8 o'clock. I read an analysis somewhere of combined pedal forces which concluded that for optimal results on a steep climb, each foot should generate force for about 220° because the forces on each pedal have a lot of taper on either end of their arc. IME that's about right. It's all about getting the muscle damage down by decreasing maximum force.