Or more specifically: use the arrow keys to move the selection rectangle over the piece you'd like to slide, then press space. Only one piece can be moved at a time, and only horizontally or vertically, never diagonally.
Or more specifically: use the arrow keys to move the selection rectangle over the piece you'd like to slide, then press space. Only one piece can be moved at a time, and only horizontally or vertically, never diagonally.