Turret Press Indexing described

The square ratchet is both free to rotate inside the clamp and held fast by the clamp, depending on which direction the ram is moving.  If you are lowering the press lever (raising the ram) you don't want the turret to rotate, so the square ratchet must be free to rotate inside the clamp.  The friction between the square ratchet and the index rod causes the square ratchet to slide down to the bottom of the cavity inside the clamp, where it is free to rotate when it passes over the twist in the index rod.  On the upswing of the press lever (as the ram goes down) the drag between the square ratchet and index rod causes the square ratchet to slide up to the top of the cavity inside the clamp, where the square ratchet fits into matching notches in the clamp which prevent the square ratchet from turning, so as it passes the twist in the index rod it turns the turret.