Dear JET,
The rear shoulder is usually the cause. In the forward stroke, the shoulder frame should work in a plane parallel left of the putt line (vertical or tilted), and this means that the rear shoulder should be heading down beneath the neck, and should NOT be moving out towards the line of the putt.
If you envision the upper torso as a cereal box atop the hips, the box should not be twisting around. The top of the box (defined by the shoulder frame) should be "rocking" beneath the stable pivot of the neck, but the shoulders (and 'top of the cereal box") should NOT be swiveling rear-shoulder out and around toward the line of the putt and the target.
So if this is indeed what is causing the loop over, out, and then inward across the line, then the cure is to work the rear shoulder under the neck in plane, along the same pathway in space that the rear shoulder initially goes up in the backstroke "rocking."
Another way of viewing the same loop problem is to observe the lead shoulder. If the lead shoulder is pushed by the outward looping rear shoulder, then the lead shoulder gets pushed back off the original square alignment of the shoulders. So another cure is to insist that the lead shoulder does not get pulled back or pushed back off the original alignment, but instead stays directly above the foot in the thrustroke.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
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