Why lying down is the point
Your neck holds up about 11 pounds, all day.
Roughly the weight of a head, carried by muscles that never clock off. Sitting makes it worse: the head drifts forward over the screen, and every inch forward multiplies what those muscles have to hold.
Lying back over a curved support takes the weight off entirely. The muscles stop bracing because there is nothing left to brace against, and the tissue is free to lengthen.
It isn’t doing something to your neck. It’s giving your neck ten minutes where it doesn’t have to do anything.