A pork shoulder is more commonly known as a large seasoned roast typical of Hispanic cultures. However, internet subcultures have recently begun to use the phrase to describe a technique of sexual nature. Typically involving a man, his wife/girlfriend, and possibly her sister. The nature of the move is a mystery, but the humor involved and the dumb looks you will get are well worth it.
"So on the PD.com forums yesterday, I heard that LMNO gave his wife the pork shoulder!"