There are a couple of ways this happens, first is when someone receives a chiropractic adjustment/mobilization. The human body releases hormones called endorphin and encephalin. These hormones are our bodies pain reducing hormones. Once you receive an adjustment, there is a neurological reflex to the muscles that are in spasm and the muscle will then start to relax.
There are also many different modalities that can be used to help reduce muscle pain such as electric stimulation, ultrasound and cryotherapy (ice). All therapy is dependent on what muscles are involved and what exactly is the cause of your muscle pain. This will be assessed during an office visit examination. I hope this answered your question.