Schematic for booting an Arduino

I remember reading an article a while ago about a simple circuit that used a sensor (probably a PIR sensor) and a transistor or MOSFET to power up an Arduino when motion is detected. It had an interesting twist as it used another component to keep another part of the circuit open so that the Arduino retains power, even after the PIR outputs 0V when there is no motion.

I can't remember what the additional component was, but the Arduino could perform its work and then shut itself down.

Does this sound familiar to anyone? I'd like to find this article or schematic to store it away, as it sounds useful. Thanks