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Originally Posted by Throttle Crazy

Actually "resistance to flow" would be considered the cause and back pressure would
be the result. Take for example a garden hose with no nozzle on the end, when it is turned on it will flow at line pressure. When you put the nozzle on the end of the hose you have just caused a resistance to flow and there for created a back pressure in the hose. Make sense? Your exhaust works the same way. If you have a resistance to flow exhaust then you will cause back pressure in the system prior to the point of that resistiction. After the restriction you will have a low pressure. If you think about how an a/c system works this principal also applies. The compressor outputs a high pressure gas which travels to the condensor. The condensor condenses the gas to a high pressure liquid. Then that high pressure liquid flows through a restriction known as the orfice tube where it is converted to a low pressure liquid on the other side of the resitriction. From there it travels through the evaporator where it is converted to a low pressure gas and sucked back into the the compressor again. The restriction is what creates that high and low pressure with the help of the compressor.

This is definitely how I was thinking about it.