Aneroid (Manual) blood pressure monitors usually consist of an inflatable arm cuff, a manual gauge (called an aneroid sphygmomanometer), and a rubber bulb for pumping up the cuff. It also requires a stethoscope to count heart rate as well. It is placed on the upper arm to be approximately the same height as the heart. Next, the bulb is pumped in order to inflate the cuff quickly and cut off circulation to the arm. The pressure then slowly releases and you listen to the artery with the stethoscope for a noise, which is actually the blood starting to pump again in your arm. At the same time, the systolic pressure is read on the dial. As the cuff continues to deflate and the noise disappears, the diastolic pressure is read on the dial.
In case of Electronic or Automatic blood pressure monitors, there is no need to inflate the cuff and hear the noise through stethoscope. They measure the pressure in the cuff which is not steady but pulsating, and take that as the systolic pressure. For diastolic pressure, the automated monitor notes the point at which the pressure in the cuff stops pulsating. The pulse measurement is made while the cuff is between the systolic and diastolic pressures. The electronics of the machine include a timer which registers the time interval between successive pressure pulsations.