This applies to many postures!
A classic is Caturanga Dandāsana or Low Plank. This is strong core-engaging posture where your neck is ideally in a neutral position, aligned with the rest of your spine.
To put your neck into extension to look forward in this posture has negative consequences on both your ability to engage your core and is very detrimental to your cervical spine.
When you put your neck into a backbend you take your head and neck out of alignment from the rest of your trunk. This means you cannot as effectively build intra-abdominal pressure. Alignment is the first principle of core engagement.
It also causes you to disengage your abdominal muscles as part of your body is already in an extension pattern. Instead you will over-contract your back muscles, which means that you lose some of the major stabilising action of your abdominals.
Also, when we take the head back without actively leading that action with our chin moving forward first, we will automatically mainly hinge at C5. This is the disc that wears out the fastest in most necks and doing this repeatedly will only accelerate that process.
Even worse is that many students jump back into Caturanga, landing with impact into this posture! Eeeks!
If you are very flexible in a forward bend, instead of lifting your head to see your toes, try to lengthen the back of your neck and the crown of your head towards your toes. You can still lift your gaze or dṛṣṭi in the direction of your toes.
The dṛṣṭi for Downward Dog is often given as the navel. The problem here is that to see your navel you need to either round your back (there’s nothing inherently wrong with rounding your low back if you use your abdominal muscles to do it) or we would have to practice a forward head position. None of us need to practice a forward head position as we nearly all have it to some degree. This head posture places a lot of stress on the muscles in the back of the neck which are consequently weakened.
Instead, you can help to correct your forward head carriage by practising Downward Dog with your ears in line with your arms and again, actively lengthening the crown of the head towards your thumbs.
Practising in this way is spine sparing. Your neck will be happier for it.
Always with you on the mat…
Monica