I know what temporary HP do: they provide a buffer, while leaving the original state of the character intact (if you receive temp HP while bloodied, you don't stop being bloodied, for example).
What I want to know is what do they represent, and if there's a general way of roleplaying them. The question arose because normal HP already represent more than just health (-2 hp doesn't imply a wound, say), and so Temp HP seem a bit redundant on the RP side of the equation.
I can understand particular cases: "A sudden favor from Moradin leaves you feeling better than you can remember, prime for battle" == +6 temp HP over your top HP, but the general case, if any, eludes me. I'd also like to know the rationalization for adding them into the game, but that's a bonus.
Best Answer
While I wholeheartedly agree with @wax eagle's answer from a personal standpoint, I don't see that sort of description used in the rules anywhere. The compendium describes Temporary Hit Points for 4e in the following way:
It is therefore helpful to reacquaint ourselves with the 4e definition of hit points.
So in essence, temporary hit points are that surge of adrenaline or burst of strength that allow you to shrug off attacks that would normally weaken you. This makes sense when you take into account the numerous sources of THP - anything from divine power1 to infernal energy2 to friendly encouragement3 to sheer martial agression4.