You might have been taught, or you teach your players to read the running back or quarterback, which is ok if you're playing against an offense from the 1990's. The game has changed so much since then getting rid of the bigger running backs and traded them for more agile and faster backs. Back then, it was just hard nose power football. But now, offenses like to spread and widen the field so they take advantage of mismatches that may occur. So reading the backfield became obsolete opposed to reading the offensive line.
Why should you read the big guys up front? Well it's simple. They will let you know not only if its a run or pass but also in a lot of cases they will take you right to the point of attack. Thus putting your defensive players in a better position to make plays. High hats equals pass and Low hats equals run; or in other words, if the offensive line raises his head high then it is usually a pass play. But if he keeps his head low then its probably a run play. The faster they can recognize if its a run/pass, the faster they will get to their responsibility and make plays. Here's an easy drill you can do with your players!