I would like to know how a 78xx series linear regulator can be used in a switching application. I have found a circuit that shows such a use, but I have trouble understanding how the circuit oscillates. Additionally, how would one choose the Zener diode?
The 470ohm and 0.5 ohm resistor to the ground terminal of the regulator give positive feedback. It operates as what is referred to as "Hysteretic mode".
Here is a link to an article on this type of converter Hysteretic mode converters.
The zener diode would only be needed if the input voltage was above the max rating of the regulator. If so the zener needs to drop enough for the regulator voltage to be at a safe level.