Dear ImbracableCrunk,
I think maybe that a certain altitude you said is 27000ft. FCOM(-800) says that Pressure breathing occurs above 27,000feet. However, I have no idea whether or not the pressure breathing means the same as you said( Above a certain altitude, it supplies 100% oxygen content at 100% pressure (the same as EMER mode). Also there is no explanation regarding the function as you said in AMM.
FL390 is from
737 Emergency Equipment.
If you can, please let me know the reason why people at the pointy end of the tube should use EMER mode above 39000ft.