I'm sorry, but what don't you understand? Pre-ignition and detonation can be caused by having an excessively lean mixture, which when compressed may burn in a less than controlled manner. Detonation (or pinking, the tinkling noise you sometimes hear under load in a car engine caused by the pressure waves hitting the cylinder walls) is the uncontrolled explosion of the fuel/air mixture rather than a controlled burn that allows the build up of pressure in the cylinder to be more gradual and less damaging.
High RPM in a light a/c causes a high load on the engine therefore you can get detonation. Piston speeds increase, the charge is heated more when the engine is running at higher RPM due to the cylinder temps being higher, therefore the vapour pressure will be lower, therefore the chance of spontaneous combustion is higher.
That make sense to you? Could you explain to the rest of us how high rpms and loads makes detonation less likely? If that's the case, my B.Eng and M.Sc will have to go back to the cracker factory I got them from.