Complex sentence: When it is moved so the notch fits over another pieces peg, the two pegged piece can be moved, once a two peg piece has been slid to the side the notched piece can be moved to its second notch and lifted off.
Split into simpler sentences: When it is moved so the notch fits over another piece's peg, the two pegged piece can be moved. Once a two peg piece has been slid to the side, the notched piece can be moved to its second notch and lifted off.