This animation demonstrates the movement of the two-toothed ratchet mechanism. The housing rocks back and forth, at the bottom of its throw the pawl clicks down and catches on the next ratchet tooth. As the housing moves back it pulls the ratchet gear with it. At the bottom of the mechanism is a second pawl that stops the gear rotating in the wrong direction.

USEFUL LINKS:
Five Tooth Ratchet Animation
Ratchet Mechanism
Ratchet Model (2)
Ratchet Model (3)