How Curl-Flat works

High-low read on the flat defender.
Outside receiver runs a SHORT curl (~5 yds, settling at the soft spot just past the LBs); slot or back releases to the flat at 0-3 yds. The flat defender can't cover both — sit on one and the QB throws the other.

Coach the details

Every player's job

Call sheet

Money downsCall on 2nd-and-medium or 3rd-and-short vs zone underneath — the flat defender can't cover both layers.
Defensive lookAvoid vs man press if your outside receiver can't separate at the top of the curl.
Game planRead the flat defender: he widens to the flat, throw the curl behind him; he sinks under the curl, dump the flat fast — it's the cheaper completion.
Money downsUse the clear-out route to hold the deep third so the curl has room, and take the backside option if the middle opens.

Pre-snap to post-snap

Cover 3 — The flats — four underneath defenders can't cover all five underneath zones (curl-flat / slant-flat)
Cover 4 — Underneath and the flats — only three short defenders (stick / snag / quick game)

Call on 2nd-and-medium or 3rd-and-short vs zone underneath — the flat defender can't cover both layers.
Read the flat defender: he widens to the flat, throw the curl behind him; he sinks under the curl, dump the flat fast — it's the cheaper completion.
High-low read on the flat defender.
Outside receiver runs a SHORT curl (~5 yds, settling at the soft spot just past the LBs); slot or back releases to the flat at 0-3 yds. The flat defender can't cover both — sit on one and the QB throws the other.

Troubleshooting

Avoid it vs tight man press when the outside receiver can't separate at the top of the curl, and vs two-deep zones with a hard-squatting corner already sitting on the curl-flat level.
It moves the sticks — it isn't a chunk play.
Curl receiver settles too deep — the LB beneath him stays in the throwing lane. Settle at 5-6 yards, not 8+.
Flat release tangles with the curl. Spacing should be at least 5 yards apart at the snap.
QB stares at the curl; if zone rotates, throw the flat fast — it's the cheaper completion.