The Weakness of the Back Rank II

Level two: the back rank has a defender. Doesn't matter — what matters is the COUNT on the mating square.

Attackers on the square > defenders of the square = the breakthrough works, even when the first piece in is your queen. You saw Qe8+!! Bxe8 Rxe8# in the first back-rank lesson. Now drill the count until it's instinct.