Just to add to what Chris has presented, I experienced the same issue that started this thread in the early days of proving out my gear during phase 1 testing.  I had two instances of the gear locking up in the air.  Funny enough it never happened while the aircraft was on jacks until I fiddled with the pressure switches to prove the point.  I did have a few things confounding the  problem such as a reversed spool in the pump and one pressure switch with hysteresis all over the map.  In any case, the clincher was instrumenting the system with pressure gauges on both sides of the circuit as described by Chris.  Things behaved exactly as demonstrated in the video.   I knew my low side pressure setting was off and the root cause but I had not gone into the detailed analysis as presented by Chris to figure out what was going on inside the pump.  Having spent some time around high powered hydraulic servo valves and associated systems in my professional career I cannot find fault with Chris's conclusions and am very grateful for his effort and insight