The basis of Korotkoff sound is related to

At the point at which systolic pressure in the aery just exceeds the cuff pressure, a spu of blood passes through with each heabeat and, synchronously with each beat, a tapping sound is heard below the cuff. The cuff pressure at which the sounds are first heard is the systolic pressure. As the cuff pressure is lowered fuher, the sounds become louder, then dull and muffled. These are the sounds of Korotkoff. The sounds of Korotkoff are produced by turbulent flow in the brachial aery. Ref: Ganong&;s Review of medical physiology 25th edition. Page:577