[MizunoJoe]

The left arm is basically irrelevant for the hitter ... the hit has no center

The only real difference between Hitter and Swinger is the physics behind the geometry - spin the flywheel or fire the right triceps. The club head always moves in an arc from any perspective, other than when the eye lies in the plane of the orbiting club head. The left shoulder is the center and the left arm has extensor action applied to it, in a proper TGM stroke, Hit or Swing. A GSEB should know this.