You know that thing in Sarvangasana where the front of your neck cramps the moment you try to draw the chin to the chest? That is not tightness. That is substitution.
The deep neck flexors — longus colli and longus capitis — are the muscles that are supposed to draw the chin toward the throat. They sit deep against the front of the cervical spine. In a desk-worker neck they have spent so many years sleeping that the sternocleidomastoid, the rope-like muscle on the front of your neck, has taken over their job. The SCM is the wrong muscle for the job. When it tries, it bulges and burns.
Jalandara Bandha — the chin lock — is supposed to be a throat seal that draws prana inward and downward. It is also a quiet test. If you cannot find Jalandara comfortably, your deep flexors are not online. The bandha is asking the deep flexors specifically. It is not asking the SCM.
The fix is small. Lying on your back, head heavy on the floor, perform a tiny chin nod — the back of the skull lengthens, the chin draws toward the throat, and the SCM stays soft. If the SCM bulges, you have gone too far. The motion is millimeters. Hold ten seconds. Repeat ten times. The fatigue you feel deep beneath the throat is the deep flexors waking up.
Two weeks of this and Sarvangasana stops being a wrestle with the front of your neck. It becomes a finish.