The red lines show the lateral aspects of the cervical vertebrae. The purple circles show the lateral aspects of C1 – notice the lack of symmetry between the two sides. Looking at the other 3D CT scans, it’s clear there’s been a rotary shift of C1 on C2.
Having watched the video, if you keep your eye on the joint between C1 and C2, as it passes round, you can clearly see the slight forwards shift of C1 on C2 (on your left) and the more pronounced backwards shift of C1 on C2 (on your right), further confirming this.