Bryan,
Upon further, and more serious reflection, your expalantion makes sense, but I think your analogy may not apply to the laws of thermodynamics or convection. However, I am not an expert in either area. Thank God! In your water through the pipe theory, you are forcing water through the pipe, actually pushing it through. But here you are not pushing air, you are pulling it by way of convection. The bbq guru theory would be more similar to your water through the pipe theory. But here, more oxygen coming in from the bottom causes more combustion, causing more air to heat up, causing the hot air to seek cold air from above the vents, causing more hot air to repalce the air that went up through the vents. Hot air always seeks cold air. Heat moves cold doesn't, cold is the absence of heat. When hot air moves out of the top vent it sucks more from below. I haven't researched this issue. But I respectfully have my doubts about your theory, even though it makes sense in your example. Regards, Mike B.