Because primary emotions are hardwired in bodily expression, activating a posture, opposite to how you feel will help you to regulate the emotion. If you think about it, you will notice that the facial expressions of happy, sad, scared, angry, and disgusted are universally identifiable. No one has to teach us to recognize them. This being the case, the pathway that leads from the emotion to the facial expression is bi-directional. This means that when you physically activate a gentle smile (not a forced smile, just gently allowing the corners of the mouth to float up), you can actually positively affect your mood.
Try it yourself. As you read, just allow the corners of your mouth to slowly float upward. Maybe even sit up a bit. Can you feel the shift? This is one of the first things I teach my clients. However, I also refer to this skill as “the breath mint for the mood.” it is a short term strategy to get through life’s frustrations, not to replace total emotional care. Click the link to read some recent research that supports the effectiveness of this strategy.