It certainly sounds to me like a measure of turn. Do they measure the arc traced by the hands on the backswing? Or is it straight-line, from address to the top?

IMO, it's pretty much useless - I can sway waaaaay back over my right leg, and leave my hands at 11 o'clock, just to increase my hand sway.

We're discussing width of swing, here. No new info - pros swings are generally wider than amateur, and pro's hands lead the clubhead through impact, while amateurs are often level or behind.