6...f5
Event #26 Returning Knights
When a players last knight is captured, instead of removing it from the board the player that captured it puts it on any empty square on the board (during the same move).
Example:
White plays Kxe2. Since it was not blacks last knight that was captured nothing special happens. The knight is just removed from the board.
Example 2:
White plays Nxf7. Since it was blacks last knight white must put the black knight on any empty square on the board. White chooses the g8 square. So the move is Nxf7@g8#
Ok.