Easy defense
Take control of your fastest defender and line him up directly in between the offense's guard and center, but a hair above where they are standing. If done correctly, you will not be called offsides when the play begins. The line of scrimmage in Madden 93 is parallel to the feet of the offensive lineman as opposed to the ball, which is why you're able to line up so close to the offense in the first place.
When the ball is snapped, you'll usually have plenty of time to blast your defender through the line untouched, and you'll be able to stop virtually every play in the game.