Refer to figure.
FLIGHT CREW OXYGEN MASK
Flight Crew oxygen masks are the quick donning type with a mask-mounted regulator.

A mask-mounted regulator supplies a mixture of air and oxygen or pure oxygen, or performs emergency pressure control. With the regulator set to NORMAL, the user breathes a mixture of cabin air and oxygen (thus in this mode, the mask will not protect from smoke and fumes), the content of which varies with cabin altitude (oxygen/cabin air ratio increases with an increase in altitude). When the cabin altitude goes above 35 000 ft. the air inlet closes and the wearer breathes 100% oxygen. The user can select 100% in which case the regulator supplies pure oxygen at all altitudes.

• If diluter demand or 100% oxygen is selected on the regulator, oxygen will flow as the crewmember inhales and shuts down for exhale.

If the situation calls for it, the user can use the emergency overpressure rotating the knob and receive pure oxygen at positive pressure. Overpressure supply is automatically started, when cabin altitude exceeds 30 000 ft.