I think squallseeker described it pretty good. In practice, watch a stereo image and then tilt your head. When you do this, the right and left image no longer align. This is normal and understandable for a recorded video. It is easier to avoid at the center of the image, by holding our head upright. However, in skybox, when you look off from center (a 180 video for example) this same affect appears. It gets worse towards the edges of the video. I can tilt my head to bring the image into focus, but it's not a natural head movement. Simply locking that axis would be a nice workaround. Locking the axis would mean we could pan the camera by turning our head up, down, left, and right, but it would ignore the head tilting left and right, which would preserve the alignment between the left and right eye. I hope this helps. VRTV was a cardboard app that had this feature. It's simply a workaround to the limitations of recorded stereoscopic movies.
