"tossin the salad while yankin the tubesteak smothered in underwear. unless there is some wanted or unwanted gas. then it's reffered to as a rusty gillespie."
In addition to the above statement, there is also the secondary humor involved with the muffled low end sound that would actually come out of a rusty trombine.

While a girl licks your ass hole, she gives you a hand job.

A rustry trombone is when a kinky german chick eats your asshole while giving you a handjob, thus going through the motions of playing the trombone. Typically, the guy is standing up and the girl is kneeling

Having a girl lick your asshole while giving you a hand job.

To play the rusty trombone is to eat a mans ass and jack him off at the same time.

when a girl tongue twisters the asshole of a man while giving him a reach around and fondles his nuts

The act of a woman giving oral sex to a mans anus while stroking his penis.