Complex sentence: If the magnet is rotated a quarter turn, the atoms will be deflected either left or right: using a vertical field shows that the spin along the vertical axis is quantised, and using a horizontal field shows that the spin along the horizontal axis is quantised.
Split into simpler sentences: If the magnet is rotated a quarter turn, the atoms will be deflected either left or right. Using a vertical field shows that the spin along the vertical axis is quantised, and using a horizontal field shows that the spin along the horizontal axis is quantised.