And from the Head butt page I learned that it is a fighting technique:
The head butt is very powerful as the entire body can be driven behind the strike. It is also potentially damaging to the person throwing the head butt. At the front of the cranium are two bony prominences, which the brain can strike into during the delivery of a head butt causing dizziness or even unconsciousness. There is also the risk of shattering the opponent’s teeth and cutting the forehead (HIV infection is a concern).
When delivering a head butt, it is wise to control the opponent’s head by holding the back of the neck. This will keep the head static and easier to strike without injury to yourself. It is also a good idea to strike above the hairline, as any abrasion of the skin will mean the blood coagulates in the hair and does not run into your eyes.
The head butt is powered by bending of the waist and driving with the legs, not by neck movement (which can cause whiplash).
The head butt can also be delivered backwards to an opponent who is holding you from behind.