hey mr wegner. great to have a Coach of your credentials here.
First let me say that I think you are a great Coach. you teach modern aspects of the game but still don't make it overally complicated. I think analyzing it too much is not going to help.
but I still do have a a couple of issues with your Videos based my studies of biomechanics literature. note that this is not a criticism as many great coaches Keep it simple and still get the results. cues and reality are not always the same and in Coaching it is not about what the Coach means but what the Student does on court:
1. you don't seem to mention the ground up kinematic chain (sequential acceleration of Body parts from the ground up starting at the legs and ending in the racket. instead you Videos seem to Focus more on the arm and racket.
2. you mention hitting "across". I do of course agree that pros do finish across but isn't that more because the circular centrifugal swing of the racket around the Body and the pronation pull the racket around?
those shots of federer seem to indicate that he does "extend" through a Little and the across happens after the ball has left the strings (which would mean that the across has absolutely Zero effect on the hit ball).
or am I interpreting this Image wrong? don't get me wrong-I still like the across (or better around! which would be a better term IMO) finish because it allows for a more free acceleration of the rackethead (the modern swing does allow for higher RHS) but I don't see a "to the ball and then across" but instead I see a constant arc around the Body which of course later turns left. or do you see the radius of the arc changing around contact?
3. the slow to contact and then accelerating Thing:
again I understand why you teach that: you want a smooth and gradual acceleration to contact and no "jerking" of the racket. but biomechanically the contact time is so short that the swing is basically over the Moment the ball touches the strings. the ball will leave the racket before the shock waves in the racket reach the Hand. there is a study about the Baseball swing that suggests that theoretically a Player could release the bat before contact and still hit the ball the same distance.
that would mean all acceleration must happen before contact (but you don't want to slow down either so your cue might be a good Thing).
Again I admire your results as a Coach and I'm certainly in no Position to criticise you but I want to understand why you teach some things. are you intentionally keeping stuff simple? of am I missinterpreting your stuff? could you please Elaborate on my 3 Points? It is probably not necessary to know all this and it would sometimes be better to just perform and athletic Task without thinking but I'm a quite analytical guy (sometimes too much