Originally Posted by Cheetah
I cite my experience in playing all my life, taking many lessons over the years, talking to fellow players, reading numerous books, reading forums and other tennis instruction sites, being a member on several paid lesson sites and watching uncountable video lessons on the serve and have never read or heard anyone say 'supinate on the serve and then pronate'.
...except for your in posts.
Here is OP & Pros Racquet Orientation Comparison
Before arm pronation phase of the serve pros always bring the racquet so the racquet string bed is parallel to the target plane and at contact racquet string bed is perpendicular to target plane. This technique allows them to use arm pronation the most efficient way. The angular path of arm pronation would be around 90°.
In case of OP serve, his racquet orientation is completely wrong, due to before arm pronation phase the racquet’s string bed is already almost perpendicular to target plane and he cannot use arm pronation with angular path around 90°, but only about 10°, see picture below.
So, what is wrong with OP slice serve?
In order to use internal shoulder rotation (ISR) the most efficient way he has to use external shoulder rotation (ESR) as much as possible. I believe he does that very well. BTW there is no waiter’s tray at all, see frames #1 and #2, so it isn’t the problem.
He has only one problem that string bed before pronation is already perpendicular to target plane.
IMO, the forearm supination is the only motion that can provide additional 80° of the racquet turn and put it in proper position – parallel to target plane. If you know any different way to do it, just tell us please.
Next picture demonstrates very clearly proper racquet’s orientations.