Here the diagram shows a predicted trajectory for underhand and overhand. In the overhand we can see there is less of an arc. With the underhand, there is a larger arc since the starting positon is much lowert than in the overhand.

In these stills we can see the different entry points of the balls. As the literature suggests, the overhand throw results in an entry point that is more centralized, and closer to the front rim. the underhand shot is aimed more towards the backboard. If the ball does not enter on the inital shot, it has a higher chance of bouncing off the backboard and going in that the overhand.