Is that backhand supination, and forehand pronation, directly related to how tight you grip your racket?
Old style, E gripped strokes used tighter grip, slower swingspeeds.
New style, regardless of exact grip, uses looser gripstrength to allow the racket head to accelerate into the strikezone, and maybe allows the racket to supinate/pronate after the ball strike.
Try it. Loose grip definitely promotes supinate/pronation.