"elbow sex" is one of the lines audience members shout in a showing of The Rocky Horror Picture Show. The line comes twice, both times Riff Raff and his sister Magenta
happen to face each other, lock hands with each other, with their arms above there heads. As they move together and at the moment all their elbows touch Magenta happens to complete a bit of overly dramatic dialogue and shouts in ecstasy. It does not look like the movie intended for the elbow touching to be viewed as sexual. Any use of the word elbow sex is likely a reference to the movie or a recreation of that event.
Two people rub elbows and make a sound of over exaggerated sexual gratification. For best effect moment should have suspense and build up and might include chanting elbow sex like is done at The Rocky Horror Picture Show.