The above is how the pose is done. The one depicted in NASM text is not recommended and if performed how they show can cause injury. Note above the student is using a blanket, neck and spine are not touching the floor. A mouse could crawl under the neck. The spine is vertically aligned and the palms face up located each side of the spine and gaze is toward the toes.
Sarvangasana, the Queen Pose of Yoga, is another asana that NASM says can be dangerous due to high stress on the neck, shoulders and spine. This time there is qualification of if exact technique is not used. However NASM does not say what that is, so they have limited knowledge in this area.
Again the neck and spine are not used to bear the load, rather use the shoulders upper arms and use hands for back support.
Inversions encourage blood flow to the brain, help move fluid of the lymphatic system. Balance and spatial awareness are developed.
Beginners should learn this pose from a qualified yoga teacher not a fitness trainer.