Referring to the lackadaisical approach of shortstopRoger Dorn from the movie Major League, the phrase is used to show one's lack of desire to initiate the romantic or sexual pursuit of another individual while simultaneously indicating that one would still be receptive if said individual was to him/herself initiate the pursuit.
"Hey, why don't you go after that one chick? She's kinda cute"
"Meh. She's on The Dorn Rule"
"What do you mean?"
"If it comes at me, I'll catch it. But I'm not going to dive for it."
When you have to pee in a movie but don’t want to miss anything, and must do the following mental equation: will the amount I have to pee cause enough discomfort to diminish my attention capacity for the remainder of the movie to an extent which is greater than the amount of the movie I would miss by making a mad dash to the restroom.
I saw Satantango last night, wonderful film but by the 5th hour I had to start doing pee calculus.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).