Now the pronation on serve is quite a missunderstood concept still i think, however most experts agree it is an end result of a proper swing and mechanics and a loose arm. Lately ive been looking at some vids of serves in slow motion and noticed a distinct difference between some servers. For example Fed, Isner and some others supinate prior to contact then pronate into contact and then fully pronate after contact so that the racquet face faces the side of the court. But some other servers, for example Djokovic or Murray they supinate and pronate into contact but then after contact have almost no more pronation amd the racquet face is more towards the ground. I have always throught that this full pronation after serve adds more pace but considering murray can hit 130mph serves and djokovic can hit fast bombs too this is quite and interesting revelation now. Not sure what to think of this now. @Chas Tennis ur the expert in serves whats ur take?