The 'count' wheel has 30 teeth with 2 of them cut deeper than the others, these are circled in red. The arm attached to the pendulum rod, circled in green, pushes against a different tooth every two seconds, moving the 'count' wheel in a clockwise direction. When this arm meets either of the two teeth which are cut deeper than the others, circled in red, it drops down and its lower end engages into the arm, outlined in blue, which is attached to the electric contacts briefly closing them.
The pendulum then reverses its swing to go back the other way, opening the contacts once again. The effect is that the contacts are closed for only a short period of time, long enough though to operate a slave clock mechanism, which will step its hands around 30 seconds.
