To expand on this, the circumference is divided into four quadrants, and you must shoot from your own quadrant without rotating the board (although physical abilities considered, many people play with house rules where you can rotate it). Saying your butt has to be on your seat limits how much you're allowed to move yourself around the board.

It's true that even with that rule, you can typically lean to take a shot from both extremes of your quadrant, however those shots are harder to aim because you're not positioned behind the shot.