Cut and cover
Two chariots take a defended palace in one combination when each has exactly one job. The a9 chariot captures the advisor with check — the left door comes off its hinge. The d2 chariot's work stays invisible until the recapture: down its own file it guards d9, so the king may never take back. And the pawn on e7 quietly owns e8, the only flight square left. That is the whole grammar of double chariots: one cuts, one covers, something seals the last exit. Cutting without covering is a blunder; cutting with covering is the kill.
Solve the challenge to finish this lesson.