Once the switch is closed capacitor charges through the 10k resistor, when the cap voltage reaches enough to push a current through the zener diode it activates the first transistor which activates the second, when the switch is opened the capacitor voltage holds the fist transistor open which quickly drains it, deactivating the second transistor thus resetting the circuit delay for next operation.
***Interested in a 555 IC circuit with similar operation***
