My two cents: Rod Cross. in Technical Tennis, Chapter 4, gives an excellent detailed discussion of spin, incident angle, closing of the racquet face, etc. Elliot, Reid, and Crespo's "ITF Technique Development in Tennis Stroke Production," Chapter 6, Forehand Mechanics fleshes out the details attractively.
It seems to me that to gain control of racquet tilt into contact, it helps to have a well-settled grip and ESR extent at launch, the beginning of forward racquet motion. Reducing randomness helps, no? Beyond that topspin can be increased three ways, by swinging faster (other things being equal), by increasing the steepness of the approach path, and by increasing forward tilt just into the hit via application of ISR (which typically increases both forward tilt, velocity upward in the plane of the stringbed, and racquet head velocity). Cross gives a good example analysis of Federer's forehand approach angle (typically about 31º for a ball above waist height, and forward tilt of about 8º....all based, of course, on a very fast basic swing speed.
I have to laugh when comments get made such as "pros don't know about or care about ISR." It is utterly untrue. They know the technical basics. They don't think about ESR because almost all of them had that built into their swing as juniors. Assuredly Fed did. They have a very grooved reliable forehand grip, which of course they can vary in emergencies. They don't hold forward tilt from the beginning, as you can see in 500 fr/sec video. The trend has definitely been to hit less steeply, but apply much more forceful ISR into contact. When you see Rafa's racquet approach the ball (at two feet out) as if it's going to contact the ball high on the frame, yet an instant later see contact is often made a bit low on the frame, you are seeing forceful ISR in action. The same for Fed and many other players.
More that these factors and techniques involves momentary adjustments to allow the usual technique to work. JMO.
