Heels off the ground and dorsiflexion for speed maintanance.
Back in the 80’s when I was teaching myself to coach I used to compare the sprinting action to something mechanical. Pushbikes used for racing bikes had thinner tyres then ordinary bikes, the reason I was given was to reduce ground drag. In a straight line a car with skinny tyres will go faster then one with fat tyres, the fat tyres are only needed for traction during hard acceleration.