The red gear is held in place via the axle, and isn't free to rotate because of the crescent shaped portion on the drive wheel. The spoke on the drive wheel catches the driven wheel before/simultaneous to the crescent rotating out of the way and freeing the driven wheel to move.
Oh wow, I see now. The green circular part pretty much locks the crescent and thus the red gear into place. That's why there is a gap in the circular part where the gear is supposed to turn. Thanks!
