It discusses the upper cervical spine of the neck, specifically C2/3, as the segment of the spine responsible for unilateral headache/facial pain which swaps sides.
Also, it is very clear to say that even if it does not swap sides- C2/3 could still be responsible.
Treatment of C2/3 for resolution of headaches is best performed with a combination of joint stretching/mobilization AND exercise to STABILIZE the segment afterwards.