Dual MOSFET design. The P- MOSFET turns on when the power button is pressed. After a second, a GPIO pin is pulled high to 3v and supplies power to the gate of the N- MOSFET. The N- MOSFET then supplies the GND for the gate of the N- MOSFET and keeps it active. Another push of the button sends a GND signal to the same GPIO pin and issues a shutdown command. When the shutdown is complete, the GPIO pin goes low and shuts off the N- MOSFET. The whole circuit is then inactive and drawing no power. Pushing the power button again will power everything back on. The switch on the right represents the GPIO status.
