Professional footwork on receive

When professionals are ready to receive, one can see them bending low before the toss and then already moving in upright while the server is still tossing the ball. I don't quite understand why they aren't staying low in the first place. Aren't they exposing themselves to being wrongfooted? Are they in good position when being that upright?

I suspect they do this to be moving while the serve comes their way, instead of being relatively immobile and low. Still I wonder why they're getting upright. Myself I stay low and try to be active on the legs.

Hi Dieter,

This is something that has happened a lot recently with increased use of the Backhand sidespin flick return.  They need to move  along way to position of that ball.

The better players are reading the position of that serve quite early by the toss of the ball and the initial swing of their opponent.  They are also so fast on their legs that if there is a late change they can adapt.

For us mere mortals, it is easier to stay down and make our movement more simple.