The rear block does far more work than anyone credits

Coaching attention goes almost entirely to the front leg, which is only half of a two-legged push.

Ask a sprinter what the back foot is for and you will usually get a vague answer about balance. That answer is why a lot of starts are mediocre.

Both legs push. The front leg stays in contact longer and produces the larger total impulse, which is why it hogs the coaching, but the rear leg is doing something the front cannot: it is applying force during the very earliest part of the movement, when the body is still low and the direction of push is almost entirely horizontal. That contribution is short. It is also the most efficiently aimed force in the entire race, because none of it is being wasted holding the athlete up.

Get the rear leg wrong and the failure is subtle. The athlete does not fall over. They simply spend a fraction longer in the blocks and come out marginally higher, and by twenty metres the deficit looks like an acceleration problem rather than a start problem. Coaches then prescribe more acceleration work. The actual fault was a rear foot that never loaded because the pedal was too far back and the knee could not get into a position to push from.

There is a related habit I would argue against, which is setting the rear pedal by rule of thumb, two foot lengths back or whatever the local convention happens to be. The right spacing depends on how long the athlete's shank is and how much force they can put out at a closed knee angle. Somebody with strong quads and short shins wants a much more bunched setup than the same height athlete built the other way, and no table of foot lengths knows that.

The way to see it is a side-on camera at ground level and a slow playback. Watch when the rear foot leaves. If it separates from the pedal almost immediately after the gun with the knee barely having changed angle, that leg contributed nearly nothing, and the athlete has effectively started on one leg.

Fixing it usually costs a few hundredths, which sounds trivial until you remember what separates a final from a semi-final.

And it is free. No extra sessions, no new equipment. Just a bit of attention paid to the leg that everybody has agreed to ignore.