A fielder pulls his hands gradually with the moving ball while holding a catch. Why does he do this?
It reduces the impact of catching the fast moving ball.
While catching a fast moving cricket ball, a fielder gradually pulls his hands backward with the moving ball. In doing so, the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the acceleration of the ball is decreased and therefore the impact of catching the fast moving ball is also reduced. If the ball is stopped suddenly then its high velocity decreases to zero in a very short interval of time. Thus, the rate of change of momentum of the ball will be large. Therefore, a large force would have to be applied for holding the catch that may hurt the palm of the fielder.