Other tips on throwing a Head Butt:
Do not ballistically snap your head into the opponent using your neck. You'll injure yourself that way. It is much more effective to tuck you chin, lower your base and use your legs to propel yourself into the opponent, much like you would with an uppercut.
Strike with the crown of your head (as Ironman noted). This way if you get cut (and if you're throwing HB out as viciously as you can, you will get cut), it bleeds into your scalp and not into your eyes.
Aim for the chin, cheek, nose or temple. Anywhere else and you're just clashing skull to skull. You want your head to make contact with a softer, more "receptive" part of their body.
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