Think of it this way:

In matters of milliseconds, the normal combustion process is a well-timed constant burn, creating cylinder pressures of around 3,000 psi or so.

Detonation is an erratic burn which happens much quicker. The peak pressure is reached before or at TDC and causes cylinder pressures of nearly 30,000 psi. The pressure is so great, it causes the aluminum piston crown to vaporize, leaving a sandblasted look or a hole through the piston. Detonation can be caused by too much ignition advance or by preignition (where combustion is started not by the spark plug, but by compression heat, ie: "dieseling").
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
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