The instrument is called a sphygmomanometer. It involves placing an inflatable
(Riva-Rocci) cuff around the upper arm where it is the same vertical height as the heart. This measuring device is attached to a mercury manometer and inflated. The pressure is gradually reduced while the medical person listens to the sharp tapping sounds called Korotkoff sounds. Without becoming too technical the diastolic and systolic pressure readings are recording while listening and measuring these sounds.