Flow augmentation:

On applying pressure to the lower limb, distal to the veins being assessed, there is increase in flow (spectral Doppler trace). Both the superficial femoral vein (image on Left) and popliteal vein (Right) show marked increase ( flow augmentation) on applying pressure below. This suggests absence of thrombosis in the intervening parts of both veins.

http://www.ultrasound-images.com/vascular.htm