“Anybody want to explain how this fits with modern ideas of heat transfer, that do not distinguish between upper and lower levels in contact. (Heat rises when air is involved since it becomes less dense from molecular motion, and rises to the top.”
The question is not how the heat transfers, its how a “bliye” or trace amount of taste transfers that according to chazal is one way, meaning that while something “exports” taste it can’t “import” at the same time, so if we pasken tatah gavar then the bottom one exports and the top imports.