Delivered from inside a grapple, a press of bodies, or any place where there is no room to draw a fist back. Like the bite it has no reach at all — but where a bite needs a mouth free, a headbutt needs only that your head can move, so it survives being held by both arms.
The skull is the heaviest bone a person has and the forehead the thickest part of it, so a headbutt lands harder than a punch. The cost is obvious: it puts your own head, and the thing inside it, into the exchange. A helm turns that trade considerably in your favour.
A rigid helm, boot or gauntlet adds 2 to the impact — which is why a gauntleted fist is worth so much more than a bare one. Thrown with an off-hand or off-foot, the strike takes −10 and loses a point of impact.