Knock resistance is an air and fuel mixtures' ability to resist rapid autoignition - when compressed in a spark ignited engine running at a specific temperature and compression ratio - long enough to avoid detonation or Engine knocking. Detonation occurs when the fuel mix not yet consumed by the flames from the spark plug autoignites all at once(or sometimes perhaps when the flame front goes supersonic because of early ignition timing). A fuel with an high autoignition temperature that burns reasonably fast and thus doesn't need early ignition timing, will therefore most often have high practical value knock resistance. Ethyl alcohol is such a fuel.
