Plant a piece on the line and the defender is cut off
A defender guards a piece or a square along a line. Drop one of your own pieces onto that line between them, and the defence is severed even though the defender is still on the board. You overlook the block because the piece is offered on a square it could be taken on.
vs game: a piece dropped onto the defender's line cut the rook off from the back rank, and mate followed.
vs puzzle: interposing on the guarding diagonal left a defended piece suddenly hanging.