The position of head makes the difference
Positioning of the head
Have a look, on both pictures she is lying and stretching backwards over the chair. The only difference is: The POSITIONING of the HEAD. Exactly this makes the difference!
Top of head resting on a block like in headstand. This stimulates the top of the brain and activates the sympathetic NS.
Shoulders and neck supported by blankets will stretch parasympathetic nerves in the neck. This activates the parasympathetic NS. The combination of backward stretch in the mid back and long neck will balance both sympathetic and parasympathetic NS
Supported head in trikonasana
Supporting and stimulating different areas of the brain
Practicing a standing pose and resting your head at the same time can be a rewarding experience. Standing poses activate your bodily systems and resting your head helps to let go mentally. Both done at the same time can be quite uplifting. You literally are practicing hatha yoga when combining the dualistic aspects of sun and moon, effort and effortlessness, yin and yang, sympathetic and parasympathetic in the present moment.
Supporting your forehead calms the mind. In this area of the brain, called frontal cortex, we plan and can think abstractly. It feels good to give it a rest.