Yeah, it's using certain muscle groups against other muscle groups.
In most of the exercises, the "upstroke" of the movement works out one muscle group while the "downstroke" works out the other group. It's all about your own body providing the resistance.
It's also a lot about flexing. Whenever you do these exercises, the targeted areas stay in a very taut state, they are being flexed. You've probably experienced flexing yourself into soreness before.
Anyway, the exercises work for the same reason that push-ups and sit-ups work. Pretty simple, really. Just try not to over-think it
