Dear god this is so painful watching the level of internet retardation on display today regarding what happens in an airliner if you suddenly dive. Go and read the bloody Boeing 737 aircraft oxygen system manual. The oxygen mask system only comes on IF the cabin pressure drops below that equivalent to being outside the aircraft exposed to the atmosphere at 14,000 feet (no, this is NOT when the aircraft IS AT 14,000 feet because that doesnt matter unless the hull is breached and equalises with the external atmosphere). Also the lower you are the MORE oxygen is in the air. It has NO controls derived from RATE OF DESCENT at all, which is bloody obvious because its a pressurised cabin, and unless its breached, flying upwards or downwards at any rate you like wont change the oxygen content of the cabin air available to the passengers. It can also be manually activated from the cabin, which, if the pilot is busy getting stabbed, is likely to be difficult (and also completely pointless unless there is a hull breach)