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Flood is a classic three-level stretch to the strong side out of the Trio bunch look, built to overload one side of the field with three vertical stems at different depths. The outside receiver runs a deep corner/go, the TE from the bunch works a flat route that settles underneath, and the boundary X on the backside runs an isolated deep post as a hot progression away from the flood. Blocking is a standard five-man pass protection with the RB releasing late into the short flat, and the primary read progression is deep corner to intermediate flat to the backside post if the strong side is capped, making this a Cover 3 beater at its core.
Key the strong side safety before the snap; if he is aligned deep middle in a single high shell, this play is designed to stress his help over the top. Watch the flat defender's depth at the snap, since Cover 3 corners sinking too deep leave a window for the underneath flat route to sit in the void. Look for user controlled defenders jumping the flat route early, which signals man coverage and should push your eyes immediately to the backside post for a one-on-one shot. Against Cover 4 or Cover 2, the corner route on the flood side gets squeezed between two defenders, so bail out to the backside post read quickly. If the defense brings a nano blitz off the edge with only four in coverage behind it, get the ball out fast to the flat route since the deep routes will not have time to develop.
Call this on third and medium to long when you expect a zone shell, since the flood concept is purpose built to attack Cover 3 rotation to the trio side. Presnap, check if the strong side safety rotates down late; if he does, throw the corner route before he can get over the top. Use the flat route as your checkdown against zone blitzes since the RB and TE both settle into throwing lanes underneath linebacker drops. If the defense shows two high safeties, switch your primary read to the backside post since single high rotation is what actually opens the corner route. Pair this play with Deep Out or Comebacks from the same set to keep defenses honest on the sideline routes, then come back to Flood once you have seen a Cover 3 shell on tape.