How to write a script to disable position and rotation tracking for VR

Hi, I am trying write a script to disable the position and rotation tracking to create temporarily losing rotation and position for the VR headset. I know there is a "usePositionTracking" option in the OVRCam object but how can i diable it when I press a key on the keyboard? Also, the VR manager file does not allowed us to manually disable the rotation anymore, but I assume we can take the inverse quaternion in order to let the "world" rotates when the use rotates his head after he pressed certain key.

Can someone help me please?