In each shoulder, there is a bone structure which holds the shoulder socket. This bone structure sits up there, held in place by muscles attached to the body's core. The bone structure can move around. Each shoulder socket can move upward (as in a shrug), downward, forward and backward, and points in between.
One can raise a particular shoulder socket by contracting muscles that attach to the bone structure and the neck and skull. This pulls the bone structure and shoulder socket toward the neck. Muscles attached to the bone structure and the ribs can contract to pull the bone structure down.
One can also move the bone structure forward and backward. The forward movement comes by contracting muscles attached to the bone structure and the front ribs. The backward movement comes by contracting the muscles attached to the bone structure and the spine.
The coordinated movement of the shoulder sockets by themselves can create a putting stroke. For example, if one shoulder socket moves forward and the other backward, the resulting movement by itself would create a partial rotation around the spinal column. If the player stood erect, this rotation would be horizontal to the ground.
If one socket moved up and the other down, the resulting movement by itself would create a partial rotation centred approximately around the mid-point between the shoulder sockets. If the player stood erect, this rotation would be relatively vertical to the ground.
A reasonable application of the shoulder socket putting stroke would see the player leaning forward, and then applying a combination of moving the lead shoulder socket to the front and down and the trailing socket back and up on the back stroke, and reversing the motion on the forward stroke, with the lead shoulder socket moving to the back and up on and the trailing shoulder socket moving to the front and down. The amount of movement of each socket would be the amount necessary to achieve a rotation vertical to the ground. All other parts of the body would not move.
In each shoulder, there are the socket up muscle groups, the socket down muscle groups, the socket back muscle groups, and the socket forward muscle groups. These muscle groups operate in both shoulders, giving us the involvement of eight muscle groups.
One could argue that the four muscle groups in each shoulder function on a coordinated basis, because the bone structure is a common element for all groups. However, the left and right shoulder sockets have the potential to move independently of each other.
There are no inherent restrictions on the movement of the shoulder sockets. Each socket can go forward and backward, up and down, and points in between.
In body terms, the muscles groups are medium sized.
In this stroke, the muscles are moving the shoulder sockets, arms and the putter.
Since only the shoulder sockets are moving, the eye sockets remain still. This is critical in this stroke, because the eyes are the mechanism coordinating the stroke and making sure that the stroke is on track.
A vertical rotation plane is not inherent in the stroke, but can be manufactured by moving the shoulder sockets in a particular way for a particular degree of forward lean. An appropriate alignment toward the target in combination with the vertical plane can get the ball on target.
The centre of the rotation will be a point mid-way between the shoulder sockets. This radius will be greater than the radius on a wrist stroke, but similar to the radius for the upper lead arm stroke.
The rotation centre will generally be steady, since the legs, spine and head are not moving. However, if the movements of the two shoulder sockets are not symmetrical in the back and forward strokes, there will be some movement of the rotation centre.
The stroke does not prevent the player from getting an eye alignment over the target line.
While the shoulder sockets can move, the movement is relatively small. The amount of movement of the socket from a neutral start position is only a few inches forward and backward. The movement up is a few inches, but the movement down is less. The movement of the shoulder sockets is amplified by the arms, so a few inches of rotation of the shoulder sockets creates more rotation at the ball. Nevertheless, the power in the stroke is limited.
The stroke should not cause issues with back health. It can accommodate a variety of degrees of forward lean, through adjustments in the way the shoulder sockets are moved.
Making the stroke work might entail the following steps:
The shoulder socket stroke can achieve a vertical rotation, with a good eye position over the target line and a comfortable degree of forward lean. However, the stroke is complicated, relying on the coordination of four muscle groups in two shoulders. The coordination comes primarily through the eyes. The complexity of the stroke approximates the complexity in hybrid strokes. The stroke is sufficiently complex that it would be difficult to try to putt by thinking about the contraction of relevant muscles at the right time.