Why a fielder in the ground gradually pulls his hands backward...

When a fielder pulls his hands backward while catching a fast-moving cricket ball, the time taken to bring the ball to rest is increased.

Since acceleration is the rate of change of velocity ($a = \dfrac{\Delta v}{\Delta t}$), increasing the time interval $\Delta t$ decreases the magnitude of acceleration.

According to Newton's second law of motion ($F = ma$), a smaller acceleration results in a smaller impact force exerted on the fielder's hands, thereby preventing injury.

Therefore, the fielder pulls his hands backward to reduce the acceleration of the ball.

Answer: To reduce the acceleration of the ball