When you are eating a girl out and the pussy just isn't enough for you. You proceed to take a quick break unknowing to your girl and take a bite of a pastrami sandwich you have hidden. Then you get back to eating the more delicious pastrami in between your girls legs. Just like George did in the episode.
Dude I was so hungry last night while I was fucking that I pulled the costanza on my girl.
When a person takes a shit and then proceeds to wipe it on the wall and leaves it for an unsuspecting person to clean it up. Its called the Helen special cause you don't see or hear it until its on the wall a la Helen Keller.
Man I stayed at this hotel last night and someone left a Helen Special for the house keepers this morning in the public bathroom.