They quantify your stress level by calculating your heartrate and the minute variations in the time between each beat, a measurement known as heartrate variability (HRV), which has been used to gauge the likelihood of mortality after a heart attack and to help assess certain psychiatric conditions.
While there are many heartrate apps available, Inner Balance is monitoring heartrate variability (HRV) - a measurement of beat-to-beat changes that occur within heart rhythms and a sophisticated and highly sensitive observation of emotional fluctuations.