When a female lying on her back lets go of the dildo she is using while it's still penetrating her, then contracts and relaxes causing the dildo to rise and/or fall back into her.
Pervert; "So, I watched some pretty sick porn last night.."
Pedophile; "Oh yeah?... what made it so good?"
Pervert; "Some porn star was doing the casper ride on the hood of a jeep!"