Searching for a fat girl's vagina to initiate intercouse while layers and layers of blubbery lard get in your way. Thus making a diving maneuver completely necessary.
Guy 1: "Yeah, dude, I totally fucked ____ last night."
Guy 2: "Isn't she like 200 pounds though?"
Guy 1: "Yeah. I was totally muff-diving but it was worth it."
To eat out a pussy with a "muff", which is a jungle of pubic hair. This can sometimes result in embarassing situations in which people tell you that you have a pubic hair or two caught in your teeth. Not reccomended for people with standards.