Why Dhoni pull hand behind after taking catch?

Dhoni is of the best cricketer not only of India but whole world. Dhoni is one of the best finisher and he is popularly known as top wicket keeper when he take catch he pulls his hand behind towards himself. This  action is not only taken by Dhoni but all wicket keeper do the same

Do you know the scientific reason behind this?

There is a physical term Momentum which may be define as the product of mass and velocity (seeped) .

When wicket keeper catch the ball they pull hand back slowly then the velocity (speed) of ball decrease after that momentum also decrease.

Due to decreasing momentum ball apply less force in the hand of wicket keeper so his hand does not get any injury. 

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