The cuff inflates to constrict the brachial artery until it is completely closed (i.e. the cuff detects no pulse, i.e. the external pressure from the cuff exceeds the maximum systolic blood pressure). Then it deflates until it detects a pulse as blood once again is able to force through the cuff's constriction (this threshold tells it the systolic blood pressure). It continues deflating until once again it detects no pulse (i.e. the minimum diastolic blood pressure exceeds the pressure exerted by the cuff).