re: "...What is meant by "backs off the timing" and how is engine temperature reduced?...

This means that the timing is retarded -- the ignition spark occurs a little later in the cycle of the piston going up and down. "Advanced" means that it occurs earlier. These timings are all in reference to the moment when the piston is at the top of its stroke.
The exact timing affects the efficiency of the combustion -- consider that, as a major oversimplification just to explain it, you might want the spark to begin to ignite the fuel/air mixture in the combustion chamber a little before the piston reaches the top of the stroke, so that, by the time the flame front from the spark spreads over the chamber, the piston will be at the top of the stroke and ready to be pushed downward by the explosion. However, this advanced timing requires a fuel that resists preignition (the measure of octane or A.K.A. value).

Engine (actually combustion chamber) temperature is affected by ignition timing (the more advanced, the hotter), and you want to avoid excessively high temperatures. BTW, in case you were wondering, this is not the same as engine coolant temperature that is indicated on the temperature gauge.