The Advantage of the WW FH is of course that you can swing harder and still Keep it in. however you have to generate that RHS in the first place.
I noticed that a lot of the WTA Players with "ATP FHs" often lack Penetration in their FH (like the italian and spanish Girls who all use the WW finish) and most hard hitting Girls use more of a swing through (against the other shoulder) finish.
We can certainly find instances of rather powerful and muscular human beings who manage to hit impressive ground strokes, regardless of their unusual technique. It is not impossible and it would seem contrary to various principles which modern psychology underlines. Certainly, you can get good at doing something wrong, bearing some limits which are imposed by physical laws and anatomical structure. The point is that there is a series of traits that a good forehand must present and it must be present at the right moment. How you manage to get them is not important, unless differences between methods happen to exist.
Here, theories meet their practical applications. Certain ways of performing tasks make it easier to duplicate them consistently, which is a primary objective in tennis. Surprisingly enough, most professional players, including top pros on both the male and female tours, do not use the most efficient way to hit a tennis forehand. The most stunning fact of this narrative is not the relative scarceness of good forehand models that we may wish to emulate in great parts, but the relative obviousness which makes the difference between both.
The key detail that I am referring occurs in between the take back and the forward acceleration; it occurs as the player first moves his racket forward. The better players pronate at this moment, whereas others supinate (that is, some of them point their palm down while others start turning their palm toward the sky). And, as usual with tennis, good technique gives you everything: more pace, more spin, more control, more consistency...
Do you know why? Those who supinate at this moment cannot manage to get a specific type of muscular reflex force their forearm into a very violent pronation around contact; those who do pronate here, will manage to get it. And, as you might have guessed, both must meet the ball with roughly the same racket head angle (with a slight forward tilt). The player who benefits of the muscular reflex closes his racket of exactly the same amount every time, with exactly the same timing... the other one must actively force his forearm into pronating.
It doesn't mean that you can't play without it. In truth, most top pros who have even won hundreds of thousands in prize money do not do it and they could get better almost instantly by learning it. These players can manage to become good at doing something wrong: they have hours and hours of practice to force a good habit into a bad technical framework... But we're amateurs and time is counted. We can't fool around for years to get our top spin forehand under control... the quicker, the better. Players that hit their forehands properly including all of the current top four (without surprise, they're also hitting some of the biggest forehands ever) and players such as Berdych, Soderlign and Roddick. On the other hand, there are players such as Gonzalez (who's retired now, if I recall properly), virtually every women on the WTA tour and, for a clear example, Hewitt. I do have one example of a woman who hits it properly: Stosur. Sam Stosur's forehand is by far the heaviest thing you will find a woman hitting -- the William sisters and others may hit with as much pace, but Stosur has both pace and spin all the time.
For curious reasons, there are flat hitters who use this trick, as noted. But they're consistently hitting great flat forehands... it's that spin is not unilaterally determined by one variable. Anyway, if you want to know why a WW forehand MAY result in more spin, it's this specific thing. What you see a "wiper" action at the end is actually a severe forearm pronation. For some players, it does work: they do get extra spin because of their beneficial muscular reflex (it's a stretch-shortening cycle, for wonks), others just fail to make it happen early enough and, so, they perform the whole thing once the ball is already gone.
As for woman, it's a symptom of a whole generation, so to speak. Rare birds get the movement right -- it's not that it's hard, it's that you have to know it or to be very lucky. You, personally, could make it happen and use it -- and you could boast that your forehand has one element which is technically more efficient than that of a GS champion (Hewitt) and it's the stretch-shortening cycle of the forearm pronators.
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