Head-Butt
Source: Pathfinder Unchained, p. 19
The monk slams his head into his enemy’s head, leaving his foe reeling. If the attack hits and the foe is of the same size or one size smaller than the monk, the monk can make a free combat maneuver check against the target of this strike (using the base attack bonus of the attack used to hit the foe). If the foe is not of the same creature type as the monk, the monk takes a –8 penalty on this check. If the check is successful, the target is staggered for 1 round. Creatures without a discernible head are not affected by this style strike (subject to GM discretion). The monk must attack with a head-butt to use this style strike.