Pressing the button will start playing beeps at a frequency of between 40 and 160 beats per minute (BPM). You will then be prompted to guess the number of beats per minute.
This is designed to help pre-hospital clinicians practise assessing a pulse rapidly, without any hardware (pulse oximeter, ECG, …), and without counting beats for a number of seconds (typically 15) and multiplying (by 4) to get the pulse (in beats per minute).