Larry Fedora's offense often packages zone reads with bubble screens on the perimeter to emulate traditional triple-option run plays. In this 3x1 (trips) package, however, the zone read is not in play. Williams only has one read to make and that's counting the box.
If there are six defenders in the box, throw the screen pass. If there are five defenders in the box, run the ball.
As you can see in this initial photo, Boston College only has five men in the box. With three defenders covering the trips set on the left side of the field, this is an easy read for Williams to make. Boston College's two safeties - Dominique Williams and Justin Simmons - are playing 15 yards deep and thus are not counted. Note how linebacker Kevin Pierre-Louis (top blue circle) and cornerback Al Louis-Jean are positioned with hips open to the ball to help in run support.
At the snap, A-back Kendrick Singleton sprints out left, effectively stretching the field horizontally by pulling Pierre-Louis toward the left sideline. On the backside of the play, wide receiver Sean Tapley stretches the field vertically by moving Louis-Jean down the field on a deep route. UNC has moved four defenders out of the middle of the field, leaving middle linebacker Steele Divitto in isolation.
A.J. Blue is lined up beside Williams at the snap, likely due to being UNC's best blocking back. Since there is no zone read option, Blue immediately charges up the field to block the middle linebacker. Note how well the left side of UNC's offensive line has blocked - left tackle James Hurst has turned his man to his outside shoulder, while left guard Caleb Peterson has turned the defensive tackle to his inside shoulder, opening a significant hole for both Blue and Williams to run through.
Here's a wide shot of how UNC's spacing has worked. Fedora's overarching scheme is designed to create vertical seams in the defense. With four defenders effectively drawn to the perimeter and Boston College's three remaining defenders (in the back seven) stacked down the middle of the field, both seams are available to attack.
This is where Williams's athleticism is put on display. While Blue has blocked the middle linebacker, the free safety has collapsed on the play to make the tackle. Williams, however, sees center Russell Bodine and right guard Landon Turner driving their block down the field and quickly changes direction to run behind his teammates.
Here's a good view of Williams's cut back to the right. The free safety is now out of position and in a foot race to make the play. Look how much open grass is ahead of Williams.
At this point, the strong safety is beginning to charge from the goal line while Tapley has his man blocked in the end zone.
Williams is finally hit at the 5-yard line and eventually brought down at the 2. Blue scored on a 1-yard touchdown run three plays later to give UNC a 13-7 lead.
Photos by J.B. Cissell