The "Donkey Punch" is a sexual maneuver performed during anal sex. The penetrating partner punches the submissive partner in the back of the head in order to induce anal contractions. The term is not strictly defined or well-documented; at least one source attests to the move being "made up."[2] One source recommends the punch take place just before the penetrating partner's orgasm, producing "heavenly" constriction of the passive partner's anus "if she passes out."[2] The loss of consciousness is found in other, but not all, descriptions.[1] Another source recommends surprising the passive partner to "add ... intensity to the clench."[3]
Source:
http://en.wikipedia.org/wiki/Donkey_punch