Truly blood pressure monitors use the oscillometric method of blood pressure measurement.
The pulsations induced by the artery are different: when the artery is compressed, no pulsation is perceived by the device, then when the pressure decreases in the cuff, the artery starts to emit pulsations: the pressure then measured on the device defines the maximal blood pressure or systolic blood pressure.
During the pressure decrease in the cuff, the oscillations will become increasingly significant, until a maximum amplitude of these oscillations defines the average blood pressure.
Then, the oscillations can still be seen during the decrease of the pressure in the cuff, until they disappear: the pressure then read on the device defines the minimal blood pressure or diastolic blood pressure.
This method of measurement of the blood pressure is the oscillometric method. It is very often used in the automatic device for the measurement of the blood pressure because of its excellent reliability.