A queen or rook with supporters forming a 3x3 box
A queen or rook delivers mate while one or two supporting pieces cover every other square in the 3x3 area around the king. The pattern looks like a box closed around the king. Common when a king has been driven to the edge and the attacker has at least two coordinated pieces.
Rook slides to a4. Mate. The rook checks along the a-file. White king cannot escape: b1 is covered by the queen on c2, b3 by the bishop on d4, a3 by the rook itself.
3 more clean runs (at any point) to reach Recall.
Black to move. White king has just taken a rook on a2 and is now stranded in the corner. Black has the rook on c4 ready to swing over.
3 more clean runs (at any point) to reach Recall.