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This KO method is based on the idea that blunt force to the head can render a victim unconscious. There are many variations on the Head KO.
A common method during the Golden Age of comics, was for the bad guy to use a device called a 'black jack'. Other methods include fists, gun butts and sword handles.
It's even possible for a Head KO to be self inflicted.
This is a very popular KO method on Sleepycomics.com.