2 Answers

The first is locking and unlocking the escape wheel. Keep in mind that this only happens at the precise moment the balance link makes it happen. That means that as long as the balance link is swinging at the link same rate, the gear train advances at extremely regular intervals. Put some hands on wheels further back in the gear train to show when the gear train unlocks, and you've got a watch.

The link second job is keeping link the balance swinging. This also happens at the moment the balance unlocks the escapement. (Escapement enthusiasts will recognize a slight simplification here, link but let's allow it for the sake of clarity). As long as the energy delivered is consistent, the balance will always swing at the same rate.