This is Wii Bowling. The game says to twist your hand to curve the ball. I was rather impressed that the ball would tend to spin left, just like real life. I figured it was because I was right-handed, and I naturally twist my wrist a bit. I was impressed by the subtlety of control. When the pins were on my right, I twisted my hand right, and it would spin right. If I used both hands, I could get it to go straight. Cool, right? The Wii controller could sense the slightest of wrist twists. Impressive. Right?
Well, given the situation above, try to make the ball spin LEFT. You can't. It's impossible. In fact, most of the twist control is just an illusion! Yes, if you use both hands, you can get it within a small range where it actually goes straight, but if you give it the slightest twist, it will always spin in the direction of the pins (within certain limits). The easy way to test this is (assuming you're right-handed) to start to the left of the pins and just bowl normally. Or even intentionally twist left. Note that the ball will now curve right, leftward twist be damned.
Within the game, this means that strikes are largely a matter of luck, but that spares are relatively easy to pick up. Beyond this game, however, it means that the Wii controller's sensitivity is probably not as good as it seems at first glance, and that they have had to fudge it quite a bit. They might also have just done this to make the bowling game easier, I guess, to make beginners happier... but at the expense of making you lose your sense of accomplishment and enjoyment once you realize how much of your score is based on luck.
Either way, it is another demonstration of the Wii control scheme's focus on "perceived performance".
Update (Jan-24-2007): I thought I'd respond to the comments below over here as well. So I did more testing, and I have noticed that, contrary to what I thought, you can control the amount of spin, so that you can make it spin more or less. However, I still maintain that you cannot control the direction of spin.
When you start a lane, hit "right" a few times so you're standing to the right of the head pin, but facing straight. Now try to get the ball to curve into the right-side gutter. It is impossible. You can only curve it left. (If you aim to the left of the head pin, you can only curve it right.) However, it is possible to curve the ball very little so it misses the head pin on the right or curve it a lot so it misses the head pin on the left.
There is more skill involved than I thought, but the direction of curve is still determined before you throw.