As the Yanks would say, here's a 'primer' on how a Diluter/Demand oxygen system works.
As the name suggests it has two functions (and settings): Diluter and Demand.
Firstly the Diluter function. The switch has two settings: Normal and 100%.
In the Normal setting the system mixes O2 with cabin air to try and maintain the pressure of O2 in the lungs that equates to sea level (ie 'Dilutes' O2 with cabin air). As cabin altitude increase, the proportion of cabin air decreases until 34 000' where the mix is 100% O2 / 0% cabin air. As cabin altitude increases above 34 000' the mix remains 100% O2, but the pressure of O2 in the lungs decreases. At 40 000' the O2 pressure in the lungs is equivalent to an altitude of 10 000'. To remain conscious above a cabin altitude of 40 000' requires 100% O2 to be delivered under pressure - this is the realm of the Pressure/Demand systems fitted to fighter aircraft and not the Diluter/Demand systems fitted to commercial aircraft.
The 100% setting on the Diluter function will deliver 100% O2 at all altitudes. It's purpose is to be used when there is smoke / fumes in the cabin so that you do not breathe any contaminated air.
Now the Demand function. The switch has two settings: Normal and Emergency.
In the Normal setting gas will only flow into the mask when you inhale (ie 'On Demand').
The Emergency setting will deliver gas to the mask continuously under positive pressure. It also, is to be used when there is smoke/fumes in the cabin. The continuous flow of 100% O2 under positive pressure ensures that if there is a poor seal between the mask and the face, O2 leaks out and not contaminated air in. Whilst the Emergency Demand setting does deliver continuous gas under a small positive pressure, it is NOT a Pressure/Demand system. There is no requirement for one and is not it's design purpose.