A clubhead's resistance to twisting when you hit the ball off-center — the physics behind "forgiveness." Higher MOI means the face twists less on a heel or toe strike, so mishits fly straighter and lose less distance. Designers raise MOI by pushing weight to the head's perimeter and rear, which is why modern drivers look stretched-out from front to back. The USGA caps driver MOI at 5,900 g·cm², and the biggest "max forgiveness" heads bump right up against that limit. If you don't hit the center every time (nobody does), MOI is your friend.