From looking at the image:
Dynamic pressure from the compressed air pushes on the inlet mass flow effective area to close the control valve. But when the control valve is closed you have no airflow, so no dynamic pressure. Static pressure inside and outside of the control valve are equal, so the spring pushes the control valve back to open until dynamic pressure and the spring force are equal. That's how that part of the valve regulates itself.
When the cabin is pressurized you have a lower pressure difference to the compressed air than you would have with the cabin unpressurized. Less pressure difference means less flow, less mass. That's why the back pressure sensing vent helps to open the control valve depending on the pressure in the cabin to adjust the flow accordingly.
I hope I got it right so far, if not please correct me.
I'm not sure what the static vent is good for. Looks like the greater the pressure difference between the cabin and the outside pressure, the less mass flow you get as cabin pressure that would help open the control valve is vented away from the back & static pressure effective area. Not sure about that.
Cabin pressure is controlled with the outflow valve(s). You always want a constant stream of fresh air to the cabin. Imagine a perfectly airtight cabin. Once it is pressurized you wouldn't need more air, so a fictional cabin pressure control valve would close. The air would be pretty stale after a 10 hour flight
So you just put in a constant amount of fresh air and dump what you don't need through the outflow valves.