[ChrisNZ]

ChrisNZ,
This sounds interesting and I would like to try it.

Could you please elaborate on the mirror position? I am assuming the mirror would be on your target line, but to the right of the ball? Or are you facing the mirror?

Thanks.

If you're in you address posture the mirror faces you, and is angled off the ground towards you. There is no ball, as it would have to sit on the mirror. A line drawn as if it were the target line could be drawn on the mirror. This represents the plane. If there was a ball, it would sit on this.

The idea is this: we have planes such as the turned shoulder plane, elbow plane and so forth. If it were possible to swing on the eye plane, the club would not appear to curve, but would trace a straight line.

Now imagine a picture of a golfer at setup viewed down the line/towards the target. Draw a straight line from the ball to their eyes. Draw another straight line to represent a selected plane angle. These two lines represent a beam of light reflecting off a surface. A mirror angled perpendicuar to an angle halfway between these two lines can be that surface. Now you can look at the mirror and 'see the [selected] plane' as a single straight line.

Hope this helps,
Chris