On Friday, Pro Football Focus writer Anthony Treash published an article addressing the NFL’s rising use of run-pass option plays (RPOs) and its leading proponent: Kansas City Chiefs head coach Andy Reid.

Noting that the use of RPOs has steadily increased in the last couple of seasons — just 4% of NFL offensive plays in 2016 to 8% in 2018 — Treash pointed out that according to PFF’s data, they also tend to be more effective; since 2016; RPOs have averaged 4.7 yards per play, while non-RPO running plays have averaged just 3.9.

And according to Treash, the Chiefs lead the way.

Over recent years, the RPO has taken the NFL by storm, and one of its largest proponents is Kansas City Chiefs head coach Andy Reid. The defending AFC West champions have led the NFL in RPO usage in each of the last two seasons (25.0% in 2018, 17.1% in 2017) and weren’t too far behind in 2016 (fourth-highest at 6.7%). It should come as no surprise then, to hear that Reid and the Chiefs are one of the NFL’s most successful teams when running these run-pass options, and they have averaged an impressive 5.8 yards per play (third-best) while doing so.

So what is an RPO?

It’s pretty much what the name implies — a play that when called in the huddle could be either a run or a pass. Either before or after the snap, the quarterback decides which way the play will go based on what he sees the defense doing.

But how can you tell an RPO has been called? Threash referenced a PFF article by Mike Renner from last year that gives an answer:

There are two obvious schematic factors occurring that are dead giveaways of an RPO: 1. The offensive line run blocks 2. Some, or all receivers, run pass routes The first means that almost all of the time, offensive linemen will be moving forward at the snap and not backwards (although that’s not the case with draw RPOs which are more popular at the college level). The latter means that certain split receivers aren’t run-blocking, even though they may still be screen-blocking. If both of those two aspects are present, you’ve got an RPO.

Renner explains that in an RPO play, quarterbacks will typically decide whether to run or pass based on the defensive coverage call (pre-snap) or what the conflict defender does post-snap. The conflict defender is a specific defensive player who is identified in the offensive play call; the quarterback simply watches what that player does after the snap. For example, if a conflict defender is a specific linebacker, the quarterback will hand the ball to a running back if that linebacker moves out wide or drops into coverage; otherwise, the quarterback will pass.

Treash said that according to PFF’s data, most RPOs end up being running plays. Since 2016, 69% of RPO calls end up as running plays, while 31% ended up being passing plays.

But Treash believes that the Chiefs’ success in RPO plays could suffer this season.

Wide receiver Tyreek Hill’s future in the NFL is up in the air, and this would be a huge loss, as Hill saw over 40% of Mahomes’ RPO pass attempts last season. On these targets, Hill earned an 82.4 receiving grade (third among wide receivers), a 130.2 WR rating, and an average of 9.4 yards after the catch per reception.

But Treash’s concern isn’t just about Hill.

Prior to his release, Kareem Hunt was Kansas City’s go-to guy on RPO plays, and he found great success as a result. Hunt’s 90.3 rushing grade on RPOs was the best in the league (13.0 points higher than second place), and his 0.29 missed tackles forced per attempt also paced the league. Furthermore, out of the 167 RPO rushing attempts run by the Chiefs, 136 of those won’t be back in Kansas City (Hunt with 112 attempts is now with Cleveland, and Spencer Ware with 24 is now with Indianapolis).

While Treash didn’t explicitly say so, this suggests Chiefs running back Damien Williams either wasn’t as effective in RPO plays, or they were rarely called when he was on the field after Hunt was released in November and Ware was injured in December.

But Treash also believes that the moves the Chiefs made in the draft were specifically intended to help remedy the situation. Sixth-round running back Darwin Thompson played in an RPO-heavy scheme at Utah State, with 101 snaps in RPOs that were good for a 81.1 rushing grade. PFF also says Thompson was one of the most elusive running backs in RPO systems, with 0.33 missed tackles forced per attempt and 4.9 yards after contact per attempt.

Second-round wide receiver Mecole Hardman played in a program that didn’t emphasize RPOs the way Utah State did. Georgia had just 30 RPO pass attempts, but Hardman got more than half of those targets and gained 9.5 yards after the catch per reception. PFF gave him a 76.6 receiving grade on his RPO snaps.