When you exit a relationship at its peak in order to avoid all the bad things associated with the downfall of a relationship. It may also be used to stay friends after a relationship.
"I can't believe Joe and Jill broke up! I thought they were so happy!!!"
"Yea well Joe pulled an HBO, he didn't think it could get any better, he was so happy, that he wanted to break it off and stay friends with Jill."
"Jen, I need to put an end to this great relationship and pull an HBO. Friends forever, right?"