A data sheet for a 750 ohm 5W 5% resistor:
http://www.parts-express.com/pedocs/specs/xicon-cement-power-resistors-spec-sheet-specifications.pdf
What does this resistor's derating chart say about what happens above 150 degrees C? Undefined or just not guaranteed (but likely to follow its linear descent to 0% load at 275 degrees C)? Or something else?
[Edit: Corrected "F" to "C"]
Best Answer
The chart is showing you how much power the resistor can safely dissipate as a function of ambient temperature. At 70 °C or below is can dissipate full rated power. Above that, its max allowable dissipation goes down until it can't dissipate anything anymore at about 270 °C.
The solid versus dotted line probably means that other specs are violated above 150 °C ambient, but they show you the full power dissipation relationship anyway, all the way down to can't dissipate anything more without getting cooked.