# What is the exact principle of this circuit? (PNP transistors and P-channel MOSFET) [duplicate]

I have this given circuit:

I can see by the look of it that this is a power rail "choosing" circuit. When VBUS is applied and there is no +5V, then the body diode of Q1 simply conducts.

I am trying to understand what the logic behind this circuit is, when VBUS and +5V are supplied externally at the same time. I think that Q1 is being turned off when +5V is higher than VBUS, but I can't explain it.

I would appreciate some hints, as I cannot really follow the scenario here with the Q2 and Q3 transistors.