Prior to contact is the most important, because that is when the velocity is being gathered and the direction set to hit the ball. It is useless to say that what happens before impact does nothing to the ball. Of course it doesn't, but the path before contact is responsible for the contact.
Clearly Nadal is meeting the ball in an arc which approaches the direction to target as the contact gets closer. That determines how the ball will be impacted.
What happens after impact is really unimportant but it is a natural consequence of the stroke and should not be under-emphasized. Just like sprinters don't come to a standstill after a 100m race, it is a physical and psychological aspect of humans.
The closing of the racket around the body also helps in recovery for the next shot. That may be a key reason which I have not seen discussed before. If you keep the racket more out in front, you are off-balance. If you turn it around, you have made yourself more compact and balanced with no floating limbs. An earlier version of this was the advice to catch the racket with the left hand after the swing, though it may not be advisable with today's swing speeds.