Now, now, Ben. This is very minor, but definitely does have to do with heat transfer. It's also something that a lot of people don't get or refuse to believe is true.
The gist of this is asking how much of the total energy contained in the fluid stream gets converted from "fluid" energy (pressure * flowrate) to thermal energy while in the block.
Personally, I don't think it matters much for a few reasons. One, at "typical" pump power levels this is relatively small vs the power of the chip(s). Two, decreasing this number generally means a less effective waterblock (though as these graphs show there is some variation in how efficiently different blocks use fluid energy). Three, there's only so much you can do and the block isn't the only thing that turns fluid energy into thermal energy.
I've said it before and I'll say it again, virtually all power you put into the pump shaft eventually turns into thermal energy that the radiator removes.
Whether you've got an engineering background or not it should not take a big leap of faith to realize that energy doesn't magically disappear. If you put energy into a pump, that energy goes into the water. If you put energy into water, you must get it back out again. At steady-state, the rate of energy entering and leaving must balance.
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