Hand signals are extremely important. Our words may not be heard. They may be misunderstood. There is no language barrier when you know the language of the hands.
The heavy machine operators can't see the huge stones they are moving. The machines are often too noisy. Conditions like these require the hands to move into action.
The quarry man is the conductor. He has an invisible score and his hands have the task of conveying precise spacial decisions at every moment of the operation so that the large chunks remain undamaged and fall properly on a cushion of rubble, landing in such a way that they don't break.