The Harry Potter is a sexual move that occurs when there are two partners participating in anal sex, and the "giving" partner pulls out but continues the anal penetration with his finger now acting as the penis unknowingly to the "receiving" partner. The "giving" partner then will fake ejaculation by spitting on the "receiving" partners back. Since the "receiving" partner assumes that the sexual activity has ceased, she/he turns around to face the "giving" partner as is typical after any sexual activity. The "giving" partner will then scream "avada kadavra", and while holding his penis like a wand he will ejaculate on the "receiving" partners face followed by taking the hand used for penetration and drawing the iconic lightning bolt scar on the "receiving" partners forehead with the feces extracted from the receiving partners anus.
Yo man I totally Harry Pottered this witch last night, and lets just say it was magical