It is held against the hub. The wheel is not turned with the gauge attached, the wheel is tuned hard left and measured, then hard right and measured.
New tricks on an old dog.
I don't need to see his video, but I thank you. I did alignments and suspension work for the vast majority of my working life. The turn plates don't have a thing to do with what I am talking about. It the spindles were not inclined and the caster was at zero, it wouldn't matter. But the spindles are both inclined (SAI) and there is a caster angle as well. Those are compound angles. Tell you what you do. Go out and start your car. Turn the wheels lock to lock sitting still. Now do it again with your foot on the brake and watch how much the front end rises on the side you turn TOO.