Final answer:
A fielder pulls back his hands to increase the time over which the ball's momentum is stopped, reducing the force and risk of injury.
Step-by-step explanation:
To reduce the impulse and prevent injury, a fielder pulls his hands back while catching a fast-moving cricket ball.
When the hands are pulled backward, the time over which the ball is stopped increases, thus reducing the force experienced by the hands due to the ball's momentum - this is analogous to reducing the 'impact' of a dive by entering the water with a streamlined shape compared to a flat-bodied 'belly flop'.
If the hands are kept still, the impact occurs over a shorter time, requiring a greater force to stop the ball, increasing the risk of injury. In the case of hitting the water, a belly flop causes a larger force than cleanly diving in, showing how distributing force over a longer time reduces injury.