The reason I teach athletes to dorsiflex the foot is because the athlete does not have to lift their knees up. It really puts the focus on what should be moving. That is the athlete's feet. It allows for a faster recovery because the athlete has shortened a lever.
The best thing about dorsiflexing your foot while sprinting or running is it allows you to forcefully put your foot back down to the ground. With the ball of the foot being the initial contact point.
Now lets go back and answer some questions.
How much should you dorsiflex your foot?
Enough so that your foot is level or toes are slightly pointing up at 90 degrees or less. To get a good dorsiflex really work n flexing the big toe up towards he knee.
Is more dorsiflex better?
NO. If the point is lifting your feet off the ground then your foot will stop traveling when your knees stops.
When do you dorsiflex?
You dorsiflex your foot as soon as your toes leaves the ground. But even if you don't dorsiflex that early be sure to dorsiflex before you foot start to travel back down to the ground.
When do you stop dorsiflexing?
As soon as you start to put your foot down to the ground.
Some athletes hold the dorsiflex too long and you give up your ability to apply maximum forces and your stride length will suffer.
The complete act of dorsflexing will allow a sprinter to put his/her foot down with much more force. This greater force application will cause the foot to rebound higher and create knee lift and a faster foot recovery time. This will assist the runner in being faster.
That is the key to why and how to dorsiflex your foot while sprinting or running.