Wind-tunnels

From TheBestCaseScenario

A wind tunnel is a specific way of driving air: you have the same intake and exhaust capability, to minimize fan problems, and air has no means to escape the tunnel. Inside, air is forced to move around the objects you placed there and can't escape, even if it doesn't like the object you placed there: a high fin density heatsink for example.

Think of a big tunnel for cars: cars enter and leave the tunnel at a given speed and if they want to get to work in time, they will speed up and down in the best way they can inside. Slower cars will be left behind (air trapped in the heatsink) and faster flowing cars will go around them, in case of air, rubbing with the slower air molecules and taking some of there heat. If there was no tunnel, no road, maybe some cars would go off-road and never rub against the hotter "cars" ;) So, cooling is improved just by not allowing air to go to waste.