I've heard it explained before, but it got so technical I got lost with some of it. But my understandiing is that in a closed system the water leaving the rad has to be the same in flow rate to it going in. However with the use of chambers and channels what is changed in the velocity of the water. With a low velocity the water it given the opportunity to disipate its heat. Hence why heater cores are so good.
If I have got that wrong I'm sure some kind and forgiving person will inform us.
