You're a chronic slicer.
Why you do it
You take the clubhead inside and behind you too quickly in your backswing. This shallow path doesn't give you much room to get the club back down past your right leg and into impact. So instead, you cast the club up over your right shoulder and down to the ball on a slicecausing outside-to-in swing path.
Swing back and through with the thin edge of a broom. Do it correctly and you'll encounter no air resistance as you swing the broom back and through on plane, but as soon as you try to take the broom back on a too-shallow plane, however, or swing it over the top on the way back down, the flat side of the broom will encounter air resistance and will immediately indicate that you're off-plane.