Foot pumps activate the 'muscle pump' in the calf, and thus improve blood circulation in the lower leg.
The heart pumps blood into the lower leg, but the return of the blood back to the heart relies on a series of valves in the calf veins. When the calf muscles are contracted the veins are squeezed the the blood is pushed up, because the valves prevent blood from going back down the veins.
In case you cannot see the video, here is a still sequence -
|Support your back comfortably on the bed with your legs outstretched. Bring the foot upwards, moving it from the ankle. Hold it up forcibly for about five seconds, and note the calf muscles will be contracted.|
|Point your toes downwards, and hold this for five seconds.|
|Repeat this sequence five times. Repeat the sequence every half hour while you're in bed or sitting up with the leg elevated.|