Cobra pose may be the most incorrectly performed movement in all of yoga. In Cobra, most people have a tendency to relax their abdominals to achieve a higher backbend. When the abdominals disengage, the head and shoulders tip forward, predisposing the low back to increased force.
A common misalignment in Cobra occurs at the start of the movement when the abdominal muscles do not engage. This initiates the movement from the low back instead, creating stress or muscle spasms across the low back. Using the “string of pearls” cue above, the correct movement needs to occur from the neck and upper back, not the low back.
(excerpt taken from The Yoga Fix)