Pulsating negative pressure applied locally, directly on the skin surface increases blood flow, probably due to a combination of dilation of the arterioles and an increased arteriovenous pressure gradient. The pulsating pressure will mechanically increase flow in the veins which have one-way valves. The reason for the pulsations is to avoid setting off the veno-arteriol reflex which constricts arteries when the veins are over-distended. That is also the reason why a constant negative pressure is counter productive.
On the figure below, one can observe in the middle panel, how blood velocities (3) increase during the negative pressure period (1), whereas there is no change in the contra lateral control arm (5) not subjected to pressure pulses.