The power switching circuit cuts off the power to the IC until it gets a high signal externally. For example, when using this circuit in a project like a PIR motion detector with Arduino, it will power Arduino when something is detected by the sensor and technically saying when the sensor sends a HIGH pulse. Here we are using this circuit in our motor driver board which will not let the power flow to the IC until a HIGH pulse is applied at the trigger pin externally saving majority of the energy while the driver is not needed.
The circuit is built around a P channel MOSFET and a couple of NPN transistors. When a HIGH pulse is applied to the circuit, transistor T1 becomes active and there is power reaching the base of transistor T2. So Gate pin of the MOSFET is pulled low and this allows the current to flow through the MOSFET and the board gets power.