Do you know what controls your heart rate, your digestion, your reproductive cycles and all the other functions of your body you don't think about? It's your autonomic nervous system (ANS). Autonomic is kind of like automatic and that's how all these things happen. Automatically without you having to think about them.
There are actually two divisions of the ANS. The sympathetic and the parasympathetic. The sympathetic is the stress response. It causes your heart rate to increase, your breathing to quicken and shallow, your blood sugar level to increase and for blood to move away from your digestive and reproductive organs. The parasympathetic does just the opposite, essentially calming you down. This is why it is called the "rest and digest" state.
The sympathetic stress response is a useful adaptation when you are in acute danger, if a tiger is chasing you, for instance. However, in a society mostly devoid of acute, dangerous stress but full of chronic low level mental stress, many of us will get stuck in the stress response. That is, we will experience things like chronic high blood pressure, chronic anxiety, chronic digestive issues, and chronic reproductive issues.
Chiropractic helps our nervous system to move into the rest and digest healing state by turning down the stress response. By aligning the spine, by getting proper motion into the spinal joints, the body naturally moves from the sympathetic to the parasympathetic nervous system. This is how chiropractic helps with stress. This is how chiropractic helps with blood pressure, digestive issues, reproductive issues and many other health problems not necessarily directly related to neck pain or back pain.