combustion chamber buildup. becomes incandescent and causes pre-ignition, fuel ignites from the hot spot and ignites from the spark plug, as the flame fronts get close the unburned fuel between the flame fronts becomes superheated and under extreme pressure and detonates
when at low RPM the time between power strokes is longer allowing time for the deposit(s) to cool, but at high RPM there is not this cool down time.
Best way to get rid of them, 1 to 2 bottles of fuel system cleaner per tank, mid to high octane gas and drive harder than normal try running gears higher than normal. Take a few trips on the highway without going to 5th gear, let the engne rev, eventually the deposits will burn away.
It may also be a sign that its time to invest in a set of new sparkplugs.
