I wish choked flow was easier to 'splain. A lot of specifying engineers seem to think it is one manifestation of the end of the earth, but it's really just a point where the basic Cv equation breaks down.
Basic Cv equation Cv=Q Sqrt ( Gf/DP)
First imagine that you set up a test stand with an infinite capacity constant-pressure source, lossless pipe, and you open the test valve valve to a fixed position. The downstream pressure starts out (with no flow) at the same pressure at the inlet. Then, start dropping the downstream pressure. As the downstream pressure decreases, flow through the fixed valve orifice will increase as the square root of the DP.
At some point, the flow stops increasing as the downstream pressure decreases. "Choked" implies that the flow experienced is less than the flow predicted by the Cv equation. When the flow measured is 2% less than predicted by the Cv equation, it is said to be "choked". Further reducing the downstream pressure with constant upstream pressure and constant valve position will cause little if any increase in the flowrate through the valve.
Here's where a lot of engineers get confused:
If you open the test valve more, you will still get more flow.
If you INCREASE the upstream pressure, you will get more flow. AND the DP will be greater before choked flow happens again.
The reference to crowding in the vena contracta was good. The flow cannot feel any more differential than the differential between P1 and where the bubbles begin to form. Similarly with a gas, when sonic flow occurs, a standing shock wave forms. The gas molecules downstream cannot communicate back upstream that there is more room to expand downstram of the shockwave, so the effective differential is between the inlet pressure and the pressure at which the shockwave forms.
Bad things happen when there is choked flow, but Choked flow itself does not mean that does not mean that bad things MUST happen.