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.