What are Korotkoff sounds?

These sounds are heard by stethoscope during the measurement of the blood pressure.
During the measurement of the blood pressure using a shygmomanometer, Korotkoff sounds are produced from under the distal half of the BP cuff. These sounds are heard between systole and diastole. They are are used to measure the systolic and diastolic BP.
When the BP cuff is inflated above the systolic pressure, pressure in cuff closes the artery (see the second arm of the previous diagram). Then the cuff is deflated slowly. And when it reaches the level of systolic pressure, this closed artery reopens with each heartbeat and creates a snapping or tapping sound (like the sail of a boat snapping in the wind).
This sound is called Korotkoff sound. The beginning of the Korotkoff sound indicates systolic BP (see the third arm of previous diagram).
As the deflation continues, Korotkoff sound disappears, at certain point (see the fourth arm of the previous diagram). This is the diastolic pressure. Here is a graph, showing Korotkoff sounds and their relation with the blood pressure.