Playbooks

Verticals out of Empty Base Flex Shotgun sends five receivers vertical at the snap with no running back protection, forcing the QB to make a quick pre-snap or hot-read decision since there is no blocking help. The outside receivers on both numbers run straight verts, the slot receiver to the trips side breaks into a deep post from inside, the other inside receiver runs a seam that bends toward the middle of the field, and the single receiver on the closed side runs a straight vertical seam. This is a classic four verts man and zone beater, with the primary read progression working from the deep post or seam matchup against the safety shell first, then working outside to the numbers verts if the middle is capped.
Read the safety alignment pre-snap, if both safeties are deep and outside the hashes you likely have Cover 2 and the seam route bending inside is open in the honey hole. If there is one deep safety in the middle of the field pre-snap, expect Cover 3 or Cover 1, and the post route working away from the safety's leverage is the primary answer. Watch for a user-controlled defender sitting in the deep middle as a lurker, since that defender can rob the post or seam if the QB stares it down too long. Key the corners at the snap, if they bail immediately into deep zones rather than pressing, that confirms zone coverage and the verts on the numbers become the outlet against soft zone. Because there is no running back, look for delayed blitzes off the edge with no protection help, and be ready to get the ball out on the seam quickly if the pocket collapses.
Check safety depth before the snap since this play lives or dies on how many deep defenders are actually in the middle of the field.
Against two deep safeties, target the seam route bending inside since it splits the difference between the corner and safety in Cover 2.
Against single high safety looks, work the post route first because it attacks the vacated half once the safety commits.
Use hot routes to convert the shallow crosser into a checkdown if you sense pressure, since Empty Base Flex has no back to help protect.
Pair this with Stick or WR Screen from the same set to keep the defense honest against underneath sag, then come back to Verticals once linebackers start cheating up.