The act of forcefully inserting ones penis in and out of a facial orifice of the human skull--hence, skullfuck.
The most socially acceptable orifice to penetrate is the mouth, as this is already a widely accepted way of engaging in sexual activity; however, the eye sockets, nostrils and even ear canals can be engaged, depending on the partners willingness to get a cock rammed into any one of the aforementioned orifices. What separates a typical blowjob from the skullfuck is the force and velocity required to achieve skullfuck status. For this, a popular scale has been invented by Aww Smeh M, measured in skullfucks (sFk).
A female performing oral sex on a male at a moderate intensity would create about 0.00001567 sFk, whereas for a typical blowjob to be considered a skullfuck, one would need to generate at least 0.005 sFk of force, which only the hips are capable of doing.
The most intense skullfuck ever recorded was performed by Chuck Norris, who skullfucked his victim so hard it generated 49 sFk and therefore vaporized his victims skull. The resulting orgasm from the aforementioned Chuck Norris skullfuck is typically referred to as an avalanche by us mere mortals.
Skullfucking usually results in the female gagging, so sufficient practice and cock-sucking experience does tend to help new practitioners to the noble art of, as the French would say, le skullfuck.
