Respiratory feedback is another type of biofeedback therapy. A randomized controlled trial from the Journal of Medical Internet Research suggests respiratory feedback is especially useful for the purposes of treating stress and anxiety. This study induced stress and compared the effects of two different treatments, splitting participants into three groups: one group used a breathing app that provided respiratory feedback, one group did a mindfulness body scan, and one group served as a control and didn't do anything.
The authors found the breathing app to be significantly more effective for the purposes of stress recovery than the mindfulness body scan. This makes sense, as there is a well-established association between breathing patterns, stress, and relaxation. Harvard Health explains that shallow breathing or "chest breathing" can trigger anxiousness, while deep breathing or "diaphragmatic breathing" can lower your heart rate and blood pressure, creating feelings of relaxation.
Another study from Cognition and Emotion describes the intimate connection between breathing patterns and emotions. This was actually two studies: The first study measured participants' breathing patterns in response to various emotional stimuli, and in the second study, participants were then instructed to practice those breathing patterns without any stimuli. Unsurprisingly, the participants' emotional states changed based on the type of breathing pattern they were instructed to practice.