The reason the racket can sometimes look like its closing quite aggressively is because its hitting the lower part of the stringbed.
Because theyre swinging so hard and with a relatively loose wrist, any impact with a ball that itself is travelling at 50mph is gonna cause a jolt with the racket face after contact, unless you physically try to lock your wrist to not allow the racket face to twist when it hits the lower part of the stringbed, which is a no no.
Now even if they catch the ball sweet or even at the top half of the stringbed, whilst the racket face will stay facing the net for longer, the racket will still close a little bit after contact because the arm has run out of room to move forwards, and so with a relatively loose wrist, the momentum of the racket is carrying on moving forwards slightly beyond your arm, pivoted around your wrist, so it looks like its closing.
But this is happening naturally theyre not forcing this, its a side effect of the stroke.