The act of a man using his ejaculation (semen) as a lubrication then performing anal on their partner to see if the 'member' exits cleanly.
The Duke being: performing the sexual act after ejaculation as a test of stamina (aka "Balls of Steel")
The Harry being: testing ones luck in prevention of excrement on their shaft (aka "feeling lucky")
Male: "So last night I was with that girl I met at the club, right? Chick wanted me to give her a Harry Duke. I was like: 'Okay, I can do that.'Needless to say: it did not turn out like I planned"