When the ball goes into the low post, trap with X1. X3 may be closer, but is on the side, not on top, and trapping with X3 would leave 3 open to cut to the basket for a pass.
Quinn Snyder - 4 on 4 shell - add an undefended coach in the other low post, he calls for the ball, double team (red) from the highest perimeter player on the top, stay locked in on the ballside feeder, the other two defenders "i up" (zone). Progression - interchange on a post feed. Lots of teams double with the opposite big. You can also use a coach as the post feeder.