From MopedWiki

Detonation, also known as preignition.

your shit's hot and the air/fuel is igniting before the spark plug sparks.

how hot?:

While death rattling hit your kill switch while still maintaining throttle position. Ideally it should instantly cut off very smoothly. But If it continues to pull you're too hot and into preignition. If it jerks violently then slows down you're into preignition but not hot enough to keep autoignition going. If it pops very very loudly once or twice(way louder than four stroking sputter) you're pipe is hot enough to ignite the unignited a/f which tells you the rest of your engine is nearly too hot. Every bike has a free head temp gauge you just gotta know how to use it.