A Harry Potter is the action of sitting on someone's face, smothering their face with the anus, and then defecating.
This is so called because the receiver of the 'Harry Potter' does not see where the feces came from (due to close proximity to the anus), suggesting the feces appeared by 'magic'.
"Last night, a chick sat on my face and gave me a 'Harry Potter'. It stunk like hell but was a bloody good laugh"
"Excuse me good sir, would you care to participate in a 'Harry Potter'?"