How does the Learjet cabin pressurisation system work? In one of the MAYDAY Air Crash Investigation episodes (about the Payne Stewart crash), the flight's emergency manual had the following information under the cabin pressurisation section:




  NOTE: At 10,000 (± 500) Feet Cabin Altitude, control pressure to the
  outflow valve is trapped. This deactivates the Automatic Mode and
  stops cabin altitude from rising higher if the failure is in the
  automatic control system.


What does it actually mean? As the NTSB investigators mentioned, it sure does sound confusing.
If the pilots know the system, it isn't confusing at all.  It simply states that a solenoid valve closes causing the cabin pressurization system to go into manual mode and the pressurization controller cannot control the cabin pressure.

It is a feature of Learjets to prevent cabin pressurization issues in case the controller is faulty.

Since you reference the Payne Stewart accident you should also know there were other issues at play that did not cause the pressurization controller to fail.  It was one of the early Learjet 35's without an emergency pressurization system.