I'll admit..I'm drawing a blank. I hear coaches all the time about using it while playing defense and now I can't remember it. Someone help me out.

Sorry, I've never heard of a "line rule". Maybe they're referring to a kind of "mercy" rule that requires the defense to stay inside the 3-point line when they're up by "x" number of points?

Maybe it dealt with the old hash mark rule! Just a thought!

AK ref SE

This would be my guess. These are coaches over the age of 45 who got into coaching when their kids started playing and they are thinking back to the days when they played, when dinosaurs roamed the earth, and small furry creatures from Alpha Centauri were REAL furry creatures from Alpha Centauri. Next time you hear it, hand the coach a current rule book and ask him to show you the rule, so you can study it up for your next game.

They may be referring to the rule regarding a throw-in when 3 offensive players line up in a row parallel to, and within 3 feet of the end- or sideline. In this case defensive players may request a spot in between and this should be granted by the officials.

Is it a Line or a Plane!!!!!!!!!!!!
hmmmmm

AK ref SE

This probably happens in about 5 games per year, but you could be right. Perhaps in the original post, this is a coach who uses that rule more than most. Thus this ref hears it more. That could be why none of the rest of us have a clue!