It seems most people get overly creative in attempting to imagine what could be meant by a sexual act entitled the "donkey punch". It's incredibly simple, while using judicious amounts of lubrication a balled fist is forced into the anus. This resembles punching the ass of the recipient although saying ass punch is more literal and less comical than the synonym donkey punch.
She told us about this freak who wanted her to fist his ass. We were horrified to hear that she donkey punched him to the elbow.