Have you tried it? The result isn't a pure topspin or slice, it's in fact, almost pure sidespin. Or atleast, the way the game renders it makes it look nothing like a traditional topspin.
I found the exact same instructions on another site, only they had correctly labeled it sidespin. A proper topspin should arc strongly downwards and bounce higher, and a slice should be slower, flatter, and bounce lower. When I try to execute a topspin in Wii Tennis, the ball just arcs left or right more strongly. Maybe it does have a slight topspin, but the sidespin aspect completely dominates.
Maybe I just suck, but I am a USTA tennis player, and I'm telling you, this game feels way more crappy than other tennis games, the rendered shots don't look like the shots you ask for. In Topspin or Virtua Tennis, I can tell a topspin or slice from the moment it leaves the racket.
And that's my problem with the Wii. The controls are too imprecise. They don't do what you ask *WHEN* you ask (lag), and because of precision issues, there is a large error rate.
But hey, let's assume your right, and that a wrist twist = spin. Well, of what use is that, when the rest of the game is so fubared? All they did was map a gesture to what is a button press in other games (or nunchuk stick move in Rockstar pingpong), nothing like the real kind of control you'd expect in Tennis or Ping Pong.
My point is, anyone whose really played tennis or table tennis, and got ahold of a controller that is supposed to model your arm movement, would expect precise control over the ball, like they would with a real paddle.
You don't get that on the Wii. And the lag issue in Wii Sports singlehandedly destroys the game for me, all other detrimental aspects not withstanding. Simply put, I do not want to *BUFFER* my next move 300 milliseconds in advance. I do not want to suffer a control system to which I must 'learn to deal with it' (the delay), when I don't have to suffer that delay on the Xbox360/PS3 equivalents.