Momentum, it uses it's own weight. Gravity brings it down, but it's heavy enough to rise up on the other side. Then coming back down, and so on. Probably has something to do with the shapes balance too.
Dude, let me point out a couple hot girl sports.
1. Cheer leading
2. Volleyball
3. Track
4. Swimming
5. Yoga
6. Gymnastics
7. You get the point.
Don't get me wrong, babes with overblown muscles are a turnoff (to me, some people like that though). But most chicks that do sports are slim...