I use video a little bit, but I'm not a big line drawer. In my opinion, the key is consistency. Try to keep the camera constant with your student from lesson to lesson. If you are comparing to a Tour Swing, make sure you are using angles as close as possible between the two swings.
Michael Breed actually wrote an excellent book on the subject...
I am really looking forward to responses from other teachers as to what they do. I would love to become more skilled in this area.
Sorry I'm not more help, great question!
Kevin
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I could be wrong. I have been before, and will be again.