M1/IC1 form the latching power switch. It draws no power when off, and little when on.
simulate this circuit – Schematic created using CircuitLab
The TLV431 (not the right symbol, this was close) is designed as a 3 terminal shunt regulator, that maintains the sense voltage at 1.24v. You could think of it as a high gain NPN transistor, with a 1.24v base threshhold voltage.
Consider what happens at the switch point. The sense voltage is around 1.24v, the supply current is passing through 200uA so there's 6v on its supply. This is within its normal operating conditions. Any tiny change in output voltage in either direction will result in switching off, or staying well latched.
I've tested the performance of the TLV431 open loop, and it's well behaved. The cathode stays above 1v, even when ref is well above 1.24v, and it comes out of 'saturation' with no hysteresis. With a cathode resistor of 33k, and 12v supply, it goes from full on to full off in about 4mV of ref terminal movement.
Note that the TLV431 is an unusual device in that the same part number from different suppliers has different specifications. The Texas device only works to 6v, so cannot be used here. The On Semi, Diodes Inc and Zetex devices are specified to 16v, so should be OK. There may be other manufacturers I have not named.
I've shown the components I use to toggle the circuit on and off. Note that for toggling off, the load voltage has to collapse far enough while C1 is holding the FET off. It has only the time constant C1.R5 to do this. If the load has a large reservoir capacitor and a low load, the time constant may need to be increased further.