How does biofeedback work?

Operant Conditioning: And learning occurs when the client is rewarded with positive audiovisual cues every time s/he changes and trends the biological measure closer to normal functioning and away from dysfunctional habits. Humans can learn to normalize blood pressure, heart rate, muscle activity (emg), stress response, and even brain wave activity (eeg) with profound effects on a wide range of common medical problems.

Biofeedback: Works by feeding back information to you about your own internal biological activity. You then use that information to learn self regulation. You can't learn any skill without feedback of your performance. Biofeedback gives you information of normally unconscious functions -- muscle activity, breathing, heart rate, sweating, skin temperature, brain waves, etc. -- allowing you to regulate them.