while engaging in doggy style sex with a bored or less than entertained partner, the act of inserting the thumb into the butthole of your partner. this will serve as a suprise to them, and will make for a more lively and entertaining experience. reactions may vary from positive to negative, but really, who cares?
Chris: dad, my sexual experience with kelsey has really become pedantic and mechanical lately.
Dad: well after you stop talking like a fag, why don't you give her the good ol' thumb start. that should get her going, it always revved your mother's engine.
Chris: ewwwww dad!