Final answer:
The strongest hand grip in a swing is required at the lowest point, where gravitational and momentum forces are greatest.
Step-by-step explanation:
The strongest hand grip during a swing is necessary at the lowest point of the swing. At this point, the forces exerted on the body and the swing are at their maximum due to the effects of gravity and the momentum of the swinging motion. A strong grip is essential to maintain hold of the swing and to prevent slipping, as well as to control the movement throughout the entire swing motion.