Defenses basically have one extra player because they don't have to match up with the QB. And defense typically uses that extra man in pass coverage (fs). They will start the game trying to stop the run with equal numbers, 7 on 7, 8 on 8. The reason they say run first, is because if they can run when the numbers are equal, it forces the defense to bring that extra man up for run support opening up the passing game. single coverage on everybody.
There are exceptions like MN and other running teams where defenses will stack the box to begin until the offense can prove they can beat the single coverage.
Originally Posted by: PackFanWithTwins