How does this PMOS circuit work?
The transistor likely contains a parasitic body diode that allows current to flow from the drain to the source even when the transistor is turned off.  It's called a parasitic device because you get it for free due to the geometry of how the transisor is constructed.  This would raise VCC5V to B_VCC minus a diode drop.  However, once BVCC5V raises high enough, the transistor wil turn on and VCC5V will be directly connected to B_VCC.  

You will sometimes see PMOS transistors installed 'backwards' to prevent conduction through the body diode, or transistors installed 'back to back'.