Thats exactly right. It has to do with velocity. If the air moving in the intake isn't moving fast, you don't get optimum volumetric efficiency (in other words you don't get as much air in the engine). By closing half the intake runners, the air on the open side moves much faster, fast enough that it rams itself into the cylinders. More air in the cylinders at low RPM means more torque. At higher RPM, that high velocity would turn into restriction, so the butterflies open, allowing more flow.
A lot of it has to do with the technical details of the inner workings of an engine. The 4 stroke engine you were taught about in school (assuming you were paying attention) doesn't exist. The valves do not open and close when the piston reaches it limits, just as the spark doesn't fire at TDC.
Let us start at the beginning of the power stroke. The mixture at this point is still compressing, the piston is moving upwards, and the spark has just fired. The flame front of the spark will spread quickly, but the timing is such that it will begin pushing on the piston around the time the piston reached TDC.
The piston moves downward from the push of the expanding burning mixture. As it moves downward, the exhaust valve begin to open. Yes thats right, the exhaust valve opens during what the textbooks call the power stroke. This causes the pressure in the cylinder to begin dropping. The gases are still burning and expanding into the exhaust. The valve doesn't burn (the way some would think), because without the pressure, this burning mixture looses a lot of heat. The fact that the exhaust is still burning and expanding as it exits the cylinder causes the exhaust to build up a lot of speed and momentum. This is an important effect.
Once the piston reaches the bottom and begins to move upward, it is not pushing the exhaust out, as pictured in textbooks. In fact by this time most of the exhaust is already gone, and the rapid movement through the exhaust system has even created a partial vacuum, drawing the piston upwards slightly. As proof on this, as the piston moves upward, the intake valve begins to open. The partial vacuum is strong enough that it even begins to draw the fresh mixture into the cylinder and starts to get the mixture in the intake moving down the runners. Some of the mixture will get drawn into the exhaust, but if the valves are timed properly, most of this mixture will remain in the cylinder.
The piston will begin moving downward, but the intake mixture set in motion earlier by the vacuum left in the cylinder moves slightly in advance of the piston, requiring minimal effort to fill the cylinder. As the piston moves, it draws in the mixture into the cylinder.
The piston reaches the end of its stroke and starts moving back upwards, but whats this? The intake valve is still open. Wouldn't that push some of the mixture back into the intake? Well, the intake charge does have momentum now, and it won't stop moving just because the piston is approaching. It continues flowing into the cylinder for a short duration longer just by inertia. The the intake valve finally closes and the mixture begins to compress. We have pretty well made it full circle. In a moment, the spark plug will fire and the mixture will burn and exert pressure driving the piston downward.
You can see, a lot of the effects going on here depend greatly on the velocity of the mixture in bot the intake and exhaust, particularly through the ports, but also through the runners in both intake and exhaust manifolds. Its all a delicate balance too. If the ports are too small, they will not be able to handle the required volume of airflow at higher RPMs and at full throttle. If they are too large, they will not be able to build sufficient inertia at low RPM and low throttle. That is why engineers have started coming out with designs like the butterfly valves on the Duratec, multiple intake and exhaust valves, some engines have variable valve timing, jet valves, hydraulic lash adjustment, etc. All to optimize the flow into and out of the engine under as many varying conditions as possible.
