When an attractive person makes direct eye contact with you and just freeze there staring and smiling; making eye contact with an attractive person and you just freeze in place blushing/smiling.
Dude, I saw this girl at the coffee shop earlier and she totes had the Medusa Effect.
"Yesterday I was rubbing one out because of boredom in the bathroom, but didn't finish because I looked in the mirror and saw the Medusa's stare."
"Oh shit, that sucks."