These engines are 'fixed timing'....this means that if you slow the RPM down significantly
the peak power pulse happes closer to top dead center.
In simple terms x amount of fuel and y amount of air are introduced through the inlet manifold and a spark occurrs at a fixed time..at normal RPMs that spark lights off the mixture with the piston at the ideal point,
so the peak power pulse happens about 16 degrees of rotation past TDC from memory, to get maximum leverage applied to the piston to spin the crankshaft which spins the prop and the knee bone is attached to the leg bone.
That mixture is igniting into a combustion chamber that is relatively large and getting larger relatively quickly as the piston accelerates downward.
If you slow the RPM the combustion chamber is relatively small, as the piston is closer to TDC, and it is not accelerating away, initially, as quickly so the Peak Power Pulse, the maximum pressure in the cylinder, is much greater and that creates more heat as well...clearing crap off your plugs and blowing it out the exhaust valve (you hope). I think it is fairly easy to understand how cycling the prop briskly (
not brutally or violently) from fine to coarse pitch at a middle of the road power setting would tend to loosen crap off the plugs.
It should also be realised that doing this at very high power settings could put you into detonation...the reason why the spark is timed to occurr when it does is because at full power on takeoff
if the peak power pulse occurred with the piston much closer to TDC the detonation margins would be reduced to near nothing...just in case someone reading this is tempted to think that if doing this at modest power settings works then doing it at high power settings works better
Hope that helps explain it novicef.
Edited to add some highlighting and restructure one sentence just a tad for jamair the pedant
