Nosedragger wins this one. Good job Nosedragger! Octane rating measure the presense of fuel compounds specifically designed to interfere with a series of low temperature (relatively) reactions leading to "combustion" in advance of the main flame front from the spark. Once the flame front is initiated, the flame front runs at about the same speed. The problem is with preignition of these end gases, you get a double pressure wave. That is the pinging we've all heard.

HOWEVER, in practice, it's a distinction with little difference. The end result is the absense of the double pressure wave makes it seem as though combustion has been slowed down, and the engine runs better. So people just say it "slows down the combustion".

So, the big question is why are car manufacturers spending millions putting anti-knock sensors in cars when this can be solved by using the correct fuel? Perhaps the reason is pollution control, and the desire to precisely control and get complete combustion.