In the beginning with a mild backfire you just hear what sounds like a very mild popping noise, usually around the intake. If you are standing beside the motor. Nothing happens to the prop or anything.

In a sevier backfire, it is almost like a big pop. The engine usually stops running.

Sometimes in the case of a sevier backfire the prop will just loosen.

Sometimes the prop and prop nuts come completely off.

Sometimes the prop is displaced with it's relationship with the spinner. Sometimes it strips off the seriations on the backplate. Sometimes it blows the spinner cone completely off.

This is usually caused by the motor instantly stopping and the prop and spinner are still trying to rotate.

A sevier backfire is caused by a lean condition and the backfire itself blows what fuel there is left out of the engine. The engine may make a revolution or two in the reverse direction but that's all.

Most highly tuned 4 strokes are highly subject to this phenomina. Likr fot example the Y-S and some of the Saito's. But I have seen OS and Magnums do it too. But the old OS 52 just usually quits and nothin happens.

Jim