A cricket player lowers his hands while catching a fast moving ball because this action helps in preventing injury to the hands. As a fast moving ball has a large momentum . In stopping (or catching) the ball its momentum is reduced to zero. Now , when a player moves back his hands on catching the fast ball , then the time taken to reduce the momentum of the ball is increased . Due to more time taken to stop the ball , the rate of change of momentum of the ball is decreased and hence a small force is exerted on the hands of player. So , they do not get hurt.