Just a small point, detonation and pre-ignition are different phenomena. It’s an interesting subject and rather than explain it myself, here’s a professionally-written link that should answer any questions:
http://www.google.co.uk/url?sa=t&source=web&cd=1&sqi=2&ved=0CBsQFjAA&url=http%3A%2F%2Fwww.contactmagazine.com%2FIssue54%2FEngineBasics.html&rct=j&q=Engine%20Basics%3A%20Detonation%20and%20Pre-Ignition&ei=ePwhTeLyF6GqhAfwrYi3Dg&usg=AFQjCNEJD3iz0SgR1feaMm6dayiYpbN5sQ&sig2=I9HKvGusHZ-dstdG0p1nUw&cad=rja
🙂