Biomechanics of Stacking
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In Jiu jitsu the stack is a powerful tool to pass or escape submissions, when used correctly.
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Correct stack positioning: the goal is to keep your weight in line with the opponent’s head, this ensures pure sagittal plane motion. When performed correctly it unlocks a fully locked in triangle, no matter how flexible their spine. .
Kinesiology of the correct stack: In an ideal spine there is a maximum of 80 degrees of cervical flexion + 45 degrees of thoracic flexion, however that is for 7 cervical segments and 12 thoracic segments. This means that each segment only has several degrees of motion, therefore you do not need too much motion to excessively load the specific spinal segment. When the specific segment reaches end range their body will activate a protective mechanism and stop the leg squeeze. Their body will not allow them to continue to squeeze and ensure that they deal with the threat of spine injury. This results in opening the legs to deload the spine.
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Even with the arms in a position that appear vulnerable to attack they will have no strength to apply an arm lock.
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Important detail I picked up from Coach @antoni_hardonk , by keeping both arms near the head it will prevent them from angling away to deload the spine in other ways.