What is elbow sex?

"It's a Transylvanian sign, you idiots!" - Patricia Quinn

In The Rocky Horror Picture Show, the characters Riff Raff and Magenta often make something like a secret handshake. They touch their hands and then their elbows together. This was apparently created by Richard O'Brien and Patricia Quinn to illustrate the bond between their characters. Since Riff's and Magenta's incestuous relationship is obvious to everyone (except Pat Quinn), fans dubbed this "elbow sex."