In practical use (for the record, no doctor is using a Speedmaster to measure heart rate. If they are, maybe consider a second
link opinion), you place your finger on a person's pulse, engage the chronograph and count to the number of pulses indicated on the
link bezel (15 or 30 on Speedmasters) at which time you stop the chrono function. The number you're left
link with gives you the average heart rate in beats per minute (bpm).