The focus is deceleration AND acceleration. The deceleration helps you stop quickly while the acceleration works on your ability to start quickly (often when needing to change direction).
The movement should be performed with high intensity.
Deceleration training is key for injury-prevention (i.e. ACL tears), as well as improving your ability to stop and change direction quickly.
Acceleration training will add the missing piece of the puzzle (i.e. once you have stopped, you need to be able to quickly change direction and have a quick and explosive start).
If you do not have a medicine ball – use a light dumbbell, instead.