This sound is caused by blood flow, which has resumed but is constricted and turbulent. At this point, the force of blood coming out of the heart is strong enough to overcome the force of the pressure cuff. This is called the systolic pressure. The pressure in the cuff continues to fall. Eventually, the stethoscope no longer detects sound.
Blood flows smoothly past the pressure cuff.
This is called the diastolic pressure. The higher the systolic pressure, the greater the force of blood coming out of the heart. The higher the diastolic pressure, the greater the resistance of the arteries to the flow of blood. Both cause stress on the circulatory system and, in particular, the heart. Below is a summary of normal and various stages of elevations in blood pressure.