These tuning knobs are interesting gizmos. They make use of a worm gear in order to allow you, the user, to tighten the string without the string being able to loosen itself. As a worm gear is rotated by a person tuning the guitar, its teeth seem to move side to side relative to the gear it is turning, pushing these teeth around in a circle. However, when the circular gear tries to push back, it is pushing perpendicular to the direction of the worm gear's rotation, almost entirely into the worm gear's teeth. The coefficient of friction between the gears is such that the frictional force between the gears always matches the rotational force generated by the contact between the two gears. As a result, it exerts no net rotational force on the worm gear.
Diagram of a worm gear.