Question

While catching A fast moving ball, a fielder graduLly pulls his hand backwards??

Answer 1

It is done to decrease the momentum of the ball, which decreases pain.

Answer 2

When a ball is caught by the hands of the fielder, the fast moving ball due to it's large force would produce great impulse if the hands are not pulled backwards gradually, which could prove to be injurious to the player.
However, if the hands of the player are gradually pulled backwards while catching the ball, the approach time increases thus decreasing the impulse, so the players usually move their hand backwards while catching the ball.

Hope this helps you !