The Whoop strap continuously measures your heart rate. However, instead of just recording the heart-rate, which is an average, is measures the time between each heartbeat (heart rate variability). I’m not an expert on the science behind it, but it’s generally thought that the healthier you are, the higher your heart rate variability (your heart can react faster to changes). Whoop uses this data to tell you how hard you worked out and how tired you are. Lebron James and Michael Phelps both use it, as do a multiple of Harvard teams.