A subject not sensitive about his or her weight stands very quietly on the scale. Observers will notice that the measurement is not extremely stable, but rather has a periodic short dip in the reading lasting a small fraction of the time between heartbeats.
The blood flows out of the heart in the upward direction. The blood to the head continues upward through the carotid artery. However, the blood servicing the lower part of the body flows upward out of the heart through the aorta, which the has a sharp bend that deflects the blood downward. The aorta exerts a downward flow on each burst of blood (called a "bolus"), which in turn exerts an upward reaction force on the aorta. This upward force is transferred to the body, which experiences a short period during which the body weight is decreased by the amount of force the bolus exerts on the aorta. This is seen in the practice as a series of small downward blips in the scale reading. Click your mouse on the link, below, to see a slow motion rendering of the weight as a function of time.
The study of this and related effects is known as "ballistocardiography." Ballistocardiography is a useful tool in determining the strength of heart muscles by directly measuring the blood flow; an electrocardiogram (ECG) measures the activity of the muscles in the heart less directly.