Can someone 'splain me POA valve vs original?

[davepl]

My very rough understanding of AC systems is that when the pressure on the high side drops below a threshold, the sensor closes a circuit that latches the clutch on the compressor.

When the pressure comes up high enough again, the compressor kicks out, or something like that.

Is that correct? If not, how does it work?

And what's the difference between that setup (which I think is original) vs. a POA valve, and how does that work?

Thanks in advance for the tutorial!