HRV

Heart rate variability is the amount of time between your heartbeats.  It is a reflection of your parasympathetic system, meaning that your body is in "rest and digest" instead of "fight or flight".  You can manipulate your HRV by controlling your breath.  This is where the biofeedback comes in - to help you know when you are in a coherent state so that you can stay there more often, reducing your stress level and helping your body to recover.