wow fluid in heat pipes, leanr something new every day, i thought they were solid. Common sense doesnt really apply to this, well doesnt to me.
Fluid in the pipes will evaporate near the hot base of the GPU (or CPU), and recondense at the cooler fins. Evaporation requires an additional input of energy (which comes from the heat-energy produced by the GPU). So really, it's just an efficient means of transporting heat energy from the bottom of the heatsink where it is concentrated, to the large fins where it can be easily transferred to the atmosphere.
This 'evaporation principle' (using the latent heat of evaporation as a cooling mechanism) is what keeps us cool in hot weather (sweatting). In a slightly different form it's also what keeps your fridge/freezer cool, and powers phase-change rigs
