Unfortunately, now that I *think* I'm figuring out how topspin forehands are hit, I can no longer try it, myself - can't lay the arthritic wrist back, anymore. Had a dandy topspin one-handed backhand, but turns out that I used a lot of flexion and extension with that shot, and can't do *that* anymore, either.
Encouragingly, using these same window pane/rainbow swing thoughts, I've been able to teach myself a two-handed backhand. It seems that the theory is robust enough that it works for that shot, too - if I remember to use the left hand for the "spin" part. Unfortunately, I remain stuck with a very ugly-looking, though somewhat effective, batch of slice forehands.
Which reminds me, I've been able to teach a number of folks how to hit topspin forehands off my massively underspun, low-skidding slices. Really good players can already handle them, but even some of the high level, local juniors couldn't get it. The key is using that same "hook" swing that Roddick likes to use - exaggerating the "across" to the right with the racket head way below the hand. BTW, without telling anyone to use a "reverse" finish with this shot, it generally naturally results from that swing.
Which also reminds me, what goes along with the rainbow/window pane stuff is that I like to differentiate between the "hand path" and the path of the racket head or stringbed. I almost think that they have to be thought of as two different beasts.
To me, the "hand path" is what determines how "flat" a swing is and how much "hit" there will be to the shot (ball speed).
The "low-to-highness" of the stringbed is largely, I theorize, the result of how fast the wrist and forearm (and whatever else might be involved) make the racket head move "in the plane of the stringbed". That rate is what Cross and Lindsey tell us is responsible for most of the "spin" (as opposed to "hit") that happens with a tennis ball.
(I know that the "slippery" poly strings are touted highly, but I've yet to see it demonstrated just how *much* additional spin they can add). I *do* know that *I* can, with my $16 per *reel*, fifteen gauge nylon can cause enough underspin with my slices that the fist bounce after it hits is *still* underspin - and *really* pisses off my opponents when warming up for a doubles match. I always explain that that's all I can do and offer to let them try to play in my wrist brace.
I'm sure this has been more than enough, but one last thought. For a topspin forehand in a "comfortable" hitting zone, I suspect that one could strike it with pretty much "pure" topspin, with little or no sidespin, having the strings strike pretty much a "straight up" path during the short time that the ball is "dwelling" on the strings. Although, it *would* be fun to talk about how "short" the "across-ness" time is of the string/ball contact that causes the "sidespin". . .