The nerve tracts of the hand are responsible for sensitivity and motor supply. The median nerve supplies the thumb, index finger and middle finger. If these are only inadequately supplied, numbness (tingling) typically occurs in these fingers. The ulnar nerve supplies the small finger and the ring finger.
Holding, steering, braking are the most important tasks that the bicycle grip must transmit - and it should be comfortable at the same time. Only a grip that fits the hand perfectly and relieves the nerves can fulfill these tasks.
The grip distance is the distance from the tip of the middle finger to the crook of the thumb. This is exactly how you enclose the grip. The grip sizes differ in diameter, length and shape.