Why a spot foul on a pass play?

The Chiefs get a nice 31-yard third down pass and run for a first down. But, it was partially negated by an illegal blindside block on a JuJu Smith-Schuster.

Partially negated? Usually when the foul is against the offense on a pass play, the enforcement is from the previous spot.

By rule, when a foul occurs after the pass is completed, it turns into a spot foul. Or if the foul occurs beyond the end of the run, the foul is marked off from the end of the run. The idea is to give the offense the yards they earned after the catch and before the foul.

On this play, the penalty was enforced 15 yards and after the walk off, the ball was behind the line to gain. So the Chiefs didn’t get a first down, they would have to repeat third down. If the walk off ended beyond the line to gain, they would have kept the first down.

That’s why the Chiefs got a 31-yard pass, got a 15-yard penalty and still get a net gain of 11 yards.