Now if you’re in the chair and the chair tilts back 30 degrees then yes, your view should tilt up 30 degrees but attached the platform is an additional tracker (or rift controller).
When this tracker registers a 30 degree shift in motion in one axis, the headset registers the same movement because everything has rolled backwards by 30 degrees even though you’re still looking forward.
So when the chair moves, we continually register the rotation of the chair and the rotation of the headset. We then subtract the rotation of the chair from the rotation of the Rift and this value is what is used by the game to determine where you are looking. So the 30 degrees movement example above returns a zero rotation. 30 – 30 = 0. Your body has rotated backwards but you’re still looking directly forward.
Now you’re tilted back 30 degrees and you look up, 10 degrees. The headset tracking registers a 40 degree rotation, but because the chair registers 30 degrees, you have a 10 degree difference.
40 – 30 = 10.