^^^ The link you've posted doesn't work, but I am assuming you are referring to the reference you have posted in http://tt.tennis-warehouse.com/showp...4&postcount=35
The supination of the left arm that Heath is referring to can be achieved by pointing the butt of the racquet at the ball, IMO. He is not talking about getting a stretch on the right shoulder, but the girl seems to be doing the right thing in the demo. I think the right shoulder stretch is important for the reason I mentioned in a previous post, and should be taught (also happens if chin rests on right shoulder at the completion of backswing). Finally, in Heath's instruction, the forward motion seems to be initiated by ISR and pronation of the left arm... IMHO, while that is a key movement, I would think that the forward motion should come from a push off the ground by the legs as the racquet is pulled towards the ball with both hands working together, and the ISR should happen as a result of hitting up and across. This makes the left hand the main contributor, but ensures that the right hand is there for more than just a ride.
Just my $0.02!