The Rules of Cypher Chess

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VII. Check, Mate, Flipping and other rules (cont.).

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f. Infiltration: A player whose King manages to move onto a Border square has the option of mov-ing any major piece (the knight, bishop, rook, or Queen but not the King or Spy) to either d0 or d10. Any piece already occupying the square is removed from the board. If the opposing King is such a piece, the game ends in victory for the infiltrating camp. (A player cannot remove one’s own Spy in this manner: this is an illegal 

move). Note that this is the only

way a player can remove an op-posing player’s Spy. Note, also: For notation purposes,moving the King onto the Border and com-pleting infiltration (going to either d0 or d10 and removing any piece sitting there) occurs as one move, though the King, in fact, moves first in the sequence of moves. Each side can complete the infiltration operation only once per game no matter how many times its King lands on the Border.​ See Illustration #34.