There are 2 problems illustrated here. First, there is a delay in the time from when the position of the user's head is read to when the GPU renders the image. Judder is when the framerate becomes uneven. For one of the frames, the GPU does not render it in time so a previous frame is shown but other frames may be rendered in time so the delay becomes uneven. This can cause nauseousness and weird behavior in the motion.
There are 2 problems illustrated here. First, there is a delay in the time from when the position of the user's head is read to when the GPU renders the image. Judder is when the framerate becomes uneven. For one of the frames, the GPU does not render it in time so a previous frame is shown but other frames may be rendered in time so the delay becomes uneven. This can cause nauseousness and weird behavior in the motion.