Short Game · Mental Model

Think of each arm as a radius that bottoms the club

The speaker frames the golf swing as a circle in which a straight arm acts as the radius that sets where the club bottoms out. If both arms are straight you get two competing radii and have to pick one; the trail (right) arm's radius bottoms out behind the ball, while the lead (left) arm's radius bottoms in front of the ball. He wants the lead-arm radius in charge, with no competition from the trail arm.