The Thundershirt works through constriction. The garment is a sturdy spandex with multiple Velcro patches that you use to tailor it to fit your dog. The constriction acts as a calming agent for your pet as dogs apparently are more calm when there is something tightly "hugging" their bodies. We put our pup in hers at the first rumble of a thunderstorm, and it GREATLY reduces her anxiety, whining, yelping, etc. It has also helped during fireworks and loud yard work. We highly recommend it to friends, and they all love it too.