Linebackers run the defense (defensive quarterbacks).
Experience.
Yeah, that's the way it's done now....
Considering that I retired the last leather helmet at our school (I kept breaking those fancy new plastic things) the only times we would bother with a defensive huddle or call signals was to warn everybody about the linebackers indulging in that new-fangled "blitz" tactic that Coach picked up watching the Chicago Bears.
Otherwise, it was pretty much a matter of lining up head on to your assigned offensive target, smack 'em upside the head when the ball got snapped and knock the socks off anybody got within arm's length of the ball. (Extra credit if you stole the ball...)
Anybody call me a Quarterback in those days was shopping for a fat lip.
Still don't have much use for them.