when a woman's partner puts a middle finger in the vagina and thumb in the anus (or vice-a-versa). rubbing the 'wall' that separates the vagina and anus from the inside with the thumb and middle finger. just like when you pick your nose, roll the booger.. then inevitably flick it.
however there is no 'flicking' when 'rolling the booger' during sex/foreplay and what have you.
My hand got so tired last night while rolling the booger with Katrina.