If you wrap something in silver foil and place it in the oven it traps the heat. That's the whole idea, so the steam doesn't evaporate and dry out what you are cooking. That's also why you put a lid on say a casserole (lol this is funny as heck)
Thus, covering the back of your card (where heat rises from because hot air is lighter than cool air, see also - hot air balloon) will prevent the heat from rising out. The parts that touch the back plate? yes, things like the ram if they are touching it will dissipate their heat as they are in direct contact. But the board itself and the back of the GPU where all the heat is rising from will simply reflect that heat back in.
TBH that may have been a part of the reason the fail rate on the 260 and 280 was so high? I have noted that my 275 does not have a back plate.
I was talking about the metal bits that do make contact with the back plate
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