When you are about to orgasm and you shout/sing "Secret Tunnel" In the spirit of Chong from Avatar and orgasm in a different orifice. The weirder the orifice the more fun had!
"We were having a good time last night and then he did The Chong in my ear."
A euphemism for female masturbation.
Melissa needs to relieve some tension, so she's 'heading down to the firing range'.