Final answer:
To signal you are going to begin your descent while diving, make a thumbs-down gesture with your hand.
Step-by-step explanation:
The hand signal used to tell the other diver that you are going to begin your descent is to make a thumbs-down gesture with your hand. This gesture indicates that you are ready to descend. It is important to use this signal to communicate effectively and ensure the safety of all divers in the water.