Alec is spot on, hit Ctrl-Numpad 4 and ASW will turn on/off by default. Ed defaults to this mode when you start the game too. It doesn't save if you change setting etc.
Dome this a thousand times but am at work and can't check;
If ASW is active, then your FPS will show a red 45fps in the ED frame counter (when you press Ctrl-F).
However in the Rift you will be seeing 90fps - you always see 90fps in VR.
ASW locks ED to generating 45fps and the Rift software creates the other 45 synthetic frames.