Warning: the following contains an explicit description of a sexual act. If you are easily offended by such things, this may be a good time to visit another node.
The donkey punch is often referred to in the context of a humorous comment, generally as an example of a callous or cruel sexual act. To perform this act, one partner must be having sexual intercourse with the other from behind. Some descriptions allow for the sex to be doggystyle vaginal intercourse, while others state it must be anal intercourse. At the precise moment of his orgasm, the partner doing the penetration punches the partner being penetrated in the back of the head. This leads to the receiver involuntarily clenching all his or her muscles (especially the sphincter in the case of anal sex), supposedly leading to a more powerful orgasm for the partner doing the penetration.
The author of this piece does not condone nor recommend this act, but since this is Everything2, it should be part of the vast catalog of knowledge here. Whether the author has ever attempted this act is left to the imagination of the reader.