The muscles of the jaw, controlling the temporomandibular joint, the joint between your jaw and your skull, is an excellent monitor to check the level of your stress, more importantly, the level of your sustained flight or fight response (sympathetic arousal).
In an article in the British Dental Journal, 2001, states facial pain and its relationship with the level of sympathetic arousal [1].
Your jaw is like your stress monitor, consciously relaxing these muscles can help to reduce the sympathetic arousal [2]. This is tough than it sounds, but it is not the actual relaxing of the muscle that helps, but it requires you to be mentally relaxed to achieve that relaxed jaw.
Reducing sympathetic arousals is important to allow your body to recover, balancing your bodies’ physiology, reducing muscular skeletal pain induced my muscle tension and postural fatigue, boost your immune system, calming the mind and many more benefits.
References:
[1] Newton-John T, Madland G, Feinmann C. (2001). Chronic idiopathic orofacial pain: II. What can the general dental practitioner do? British dental journal 191pp.72–73
[2] Schmidt JE, Carlson CR. (2009). A controlled comparison of emotional reactivity and physiological response in masticatory muscle pain patients. Journal of orofacial pain 23(3) pp-230-42.