This skill allows you to follow up a successful attack with a vicious headbutt to your opponent. The damage done by the headbutt is increased if you're wearing any kind of turtle as a helmet.
The power doubling from Tao of the Terrapin does apply to the damage from this skill.
If the numeric power of the hat is odd (such as the samurai turtle helmet or irate sombrero), the doubling will add +1 to the upper end of damage. A hat of 45 will do 9 to 19 damage, for example.
Other butting skills can be used in combination with this skill, but only as a Turtle Tamer:
With Kneebutt, you can do a Head + Knee Combo for 8 MP.
With Shieldbutt, you can do a Head + Shield Combo for 9 MP.
With both Kneebutt and Shieldbutt, you can do a Head + Knee + Shield Combo for 13 MP.
If you have a ranged weapon and you use this skill you get the following message:
You can't use Headbutt with a ranged weapon. Because, y'know, that's not how ranged weapons work.
References
Headbutting is a fighting technique that uses the cranium to strike an opponent to cause injury or a knockout.
