Your veins normally circulate blood by using the normal skeletal muscle contractions in combination of a series of one way valves. In essence, it is a passive s

Your veins normally circulate blood by using the normal skeletal muscle contractions in combination of a series of one way valves. In essence, it is a passive system. By applying constant external pressure, you shift the balance of blood flowing between your surface and deep veins (your deep veins carry way more blood). The increase in pressure also increases your blood velocity. Here's a linky http://www.ncbi.nlm.nih.gov/pubmed/10633254 http://www.ncbi.nlm.nih.gov/pubmed/10633254