A pro set formation in football is an offensive formation in which two running backs line up side by side in the backfield behind the quarterback, along with a wide receiver on either side of the offensive line near the sideline. The running backs typically line up over the B gap on either side of the ball. Pro set offense is also referred to as split backs. In this particular formation, the quarterback takes the snap under center. There also exists a number of variations including spread or two tight end sets.
Pro set is a great formation for a run-heavy offense with two good running backs and a quick offensive line. It relies on a lot of misdirection and pulling plays to gain yardage. More specifically, pro set is a formation primed for counters, read options, and iso plays.
Read options, much like a read option in any other formation, entails a play that has multiple outcomes. For example, in a triple option blast play, the quarterback could hand the ball off to one running back, pitch the ball to the other running back on a sweep, or keep the ball himself and run an off tackle play. This often involves reading one or more defensive players, usually the end or outside linebacker.
Lastly, iso plays involve "isolating" one defender and utilizing one of the running backs as a lead blocker who'll take the iso man head on. These plays rely on much less misdirection than the other two archetypes, but they succeed thanks to their powerful blocking schemes.
A smart defensive coordinator knows that much of the pro set offense relies on zone blocking rather than man blocking, a key component that can be exploited by quick inside linebackers. The 4-3 defense works best at wreaking havoc on this zone block scheme by utilizing a unique collaborative strategy between the middle linebacker and the nose guard. Essentially, the nose lines up in a 2i or "G" technique, where he fills the weak side A gap and the Mike plays head up on the center, allowing him to move into either A gap. This takes away the inside run by forcing the center to over commit to the block, allowing the Mike to slip into either A gap.
The direction the Mike goes is determined by the first step taken by the center after the snap. This is referred to as a "belly key." This very simplistic strategy takes away most inside run plays, and beyond this stopping the rest of the run game relies only on a couple of disciplined ends and outside linebackers who read the ball and the offensive linemen. A good end will stay home in anticipation of a counter, and linebackers who are keen on iso plays will move up to fill the hole before the lead blocker makes it to the line of scrimmage.
Once the run game is shut down, so is the pro set offense for the most part. Like most two running back formations, pro set sacrifices a diverse pass game for a strong run game. This means that once the run game fails, it's time to change formation.