Systolic blood pressure
The blood pressure cuff is placed on the upper arm and inflated to stop blood flow from the brachial artery to the arm. When the pressure in the cuff exceeds the pressure in the artery, blood flow is occluded and no pulse will be felt below the cuff. As you slowly release the pressure cuff, the pressure begins to decrease. When it reaches the peak systolic pressure, the artery opens a little but the flow is turbulent rather than laminar. This generates tapping sounds called Korotkoff sounds, which can be heard through a stethoscope. When turbulent blood flow is first heard, the cuff pressure approximates systolic pressure.
Diastolic blood pressure
As the cuff pressure continues to decrease and the artery regains its normal diameter, blood flow becomes laminar and the sounds become muffled and then disappear. The cuff pressure at the point of the sound muffling approximates the minimum blood pressure (diastolic pressure). However, the disappearance of sound is commonly used to determine diastolic pressure as it is easier to detect than muffling sounds. This is acceptable since the two occur within a few millimeters of mercury.
Note that in some healthy people, sounds can be heard at pressures appreciably below the true diastolic pressure. In these people, it is not possible to define their diastolic pressure accurately.