Rusty trombone
Rusty trombone is a sexual act in which a man stands with his knees and back slightly torqued with feet at least shoulder width apart to expose his anus. A person typically kneels behind the man and performs anilingus while reaching up beneath the scrotum or around the body to manually administer rapid back-and-forth motions of the penis, mimicking the motions of a trombone player. The act is defined primarily by the physical orientation of the partners, the combination of anilingus with manual sexual stimulation and the resemblance of the anal sphincter to a trombone mouthpiece; however, other positions and variations are possible.
https://en.wikipedia.org/wiki/Rusty_trombone