This can be effective during times of close combat or times when your limbs are restricted. However, head butting is dangerous if you don't aim accurately or if your opponent suddenly moves. When you head butt, you want to use your forehead to hit your opponent square in the nose. Be very careful not to hit their forehead, which could cause more damage to you then to them.
Completely bend your arm at the elbow and swing your elbow in an upward arc, or swing across your body while rotating your shoulders in the direction you are swinging.
This strike is most effective when the target is guided into the thrusting knee. If a person is struck in the stomach or groin they should naturally bend over. You can then thrust your knee up into the face. You can also pull the target down using one or both of your hands. You might want to fake a grab with one hand and grab with the other. Or eagle claw with one hand and grab with the other. With one or two hands, pull the target down while thrusting one of your knees up into it. When you thrust your knee extend your toes to add momentum. Follow through. Ouch!