[Weightshift]

Good point, Weightshift. I could have been a bit more precise with regards to the Left Arm, but the question I was answering with the drill focused on Right Arm and Elbow Action and the muscular activity used to motivate them. And that action will be unaffected by a Left Arm that bends a bit.

It doesn't just bend a bit, it bends a lot.

You wrote..

To understand this action, simply extend your Left Arm directly in front of you about chest-high. Grasp your Left Wrist with the thumb and forefinger of your Right Hand. Then, using only your Right Arm (no Left Arm activity) proceed to lift and lower the Left Arm to your heart's content. Notice that the Left Arm is completely passive and the 'work' is being done with the bending Right Elbow (via the Biceps) as the Right Deltoid (Shoulder muscle) lifts the Right Arm. The only differences between this action and your actual Backstroke are (1) you are lifting and lowering in a vertical plane in this drill versus an Inclined Plane in your Golf Stroke and (2) there is no Pivot in the drill and there is in the actual Stroke.

I held my Left Arm out in front of me, chest high, with the Left thumb pointing upwards, grasped the Left Wrist with the thumb and forefinger of my Right Hand, right thumb on top, right forefinger below.

Bending the Right Elbow to raise the Left Arm, causes the Left Arm to bend at the Left Elbow in unison with the bending of the Right Elbow. Try it.

Even with Extensor Action i.e. pulling the Left Arm straight out in front of you, with the thumb and forefinger of the Right Hand, it is difficult to prevent some bending of the Left Arm at the elbow.

Another problem is that the Right Forearm is moving in a circular motion, up and down, from level, and the passive Left Arm automatically responds to this shortening radius by bending the Left Elbow, as it has nowhere else to go.

Personally I believe that this exercise is better done with the Left Arm positioned across the chest to the Right, pointing to about 2 o'clock (body facing 12 o'clock). Now exert Extensor Action, the pulling of the Left Wrist towards 2 o'clock by the thumb and forefinger of the Right Hand, at the same time bracing the Left Shoulder against this pull (the Left Tricep action you describe) -- a two-way stretch.

Now the circular motion of the Right Arm as it bends at the Right Elbow, and lifts and lower the Left Arm, allows the Left Arm to also describe a circular motion and resultant bending at the Left Elbow is minimal.