With the output of the regulator at 5 V, the voltage at the cathode of zener D1 will be approximately 11 V. The MOSFET has a high-value pull-up resistor (R1) to VBATTERY, which turns on Q1 but keeps it operating in its linear region. Because the 2N7002 has a 1-V threshold voltage, the voltage at its source will be around 10 V. That's 2 V less than the maximum input voltage of the regulator.
The value of gate resistor R1 allows the circuit to supply the minimum load current at the maximum input voltage. Otherwise, as the input voltage increases, the current flowing through R1 and D1 would be higher than what the load is demanding, and the output would start to rise, taking it out of regulation. The value of resistor R1 is determined by:
RMIN = \[(VIN max) - (VZ + VOUT )\]/IMIN
Because MOSFETs are voltage-driven devices, almost no current is used to keep the gate turned on. What's more, because all of the current to set up the zener voltage on the gate is fed directly to the load, there's no wasted current going to ground.With this circuit, it's possible to produce 50 mA of output current with less than 5 mA of quiescent current. The only precaution needed is to monitor the heat generated by the MOSFET. Because Q1 is dropping all of the voltage above 10 V, it will dissipate some power. The higher the output current, the more heat will be generated.