more of a conscious thing in my opinion...a sprinter driving into the track will tend to drop their hips rather than keeping them high and driving forward.
Snapping down quick off the ground contact, yes. Don't need to force the leg down to the ground, gravity takes care of that. Force comes from pushing off the track not driving into the track.
Think of a dorsiflexed foot as a loaded gun ready to fire immediately upon ground contact. Think of a plantar flexed foot as a gun that needs loading during the ground contact in order for it to be able to fire. Less time on the ground > quicker ground contacts > ground contact remains underneath the hips > more efficient stride > faster