one plane teaching or instructions online.
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as a "one plane swing" can mean different things to different people it might help to define what you mean by that.
as for online instruction, i think youll find many many people here who will vouch for Evolvr. i personally have not done it but my interaction with the guys on this site who run Evolvr has led me to the conclusion that they are at the top of the field as far as research based golf instruction.
My own swing has been built on one plane teaching. I actually became aware of it through golflink and Mike Labauve. That led to me buying Jim Hardy's book which gave me the worded concept more than anything and the technical do's and dont's.
For the visual info found out that Jeff Ritter was taught by Hardy and he has lots of free videos on Youtube and I have written to Jeff and he does offer online coaching if that's the route you want to go.