A sphygmomanometer works using a balance principle - an air-filled pressure cuff wrapped around the arm compresses the brachial artery to a point where blood can no longer flow. The examiner slowly releases the air from the cuff and uses a stethoscope to listen for the return of blood flow. At the balance point, where pressure in the cuff equals systolic artery pressure, a ‘whooshing’ noise can be heard as blood flows through the artery again.