This is a typical slave unit found in a great many UK slave clocks, that use a thirty second impulse. This particular model is made by Gents, another common maker being Synchronome. Although they are both different in appearance, the principle of operation is the same for both of them.

The electro magnet attracts the armature, which takes the form of a driving arm with a pawl on the end that engages a tooth of the large central wheel each time the arm operates. As the clock is designed to work on 30 second pulses, there are 120 teeth on the central wheel. The wheel's shaft has the minute hand connected directly to its end. The hour hand then running off this shaft in a conventional manner using a Cannon Wheel.

Every time the magnet operates, the arm's driving pawl is pulled backwards and drops behind the next tooth. When the magnet releases the arm it is then restored by a spring to its original position. This restoring action cause the driving pawl, which is now wedged behind a fresh tooth, to push the wheel in an anti-clockwise direction, until the pawl comes against the 'Forward Stop' post.
As the wheel rotates anti-clockwise the backstop pawl is lifted up by a tooth passing under it and then engages behind the next tooth. The pawl's purpose being to stop the wheel being pulled back on itself when the driving pawl is activated by the magnet at the very start of the cycle.