Complex sentence: Forward-slip is used to steepen an approach (reduce height) without gaining much airspeed, the Sideslip moves the aircraft sideways (often, only in relation to the wind) where executing a turn would be inadvisable.
Split into simpler sentences: Forward-slip is used to steepen an approach (reduce height) without gaining much airspeed. The Sideslip moves the aircraft sideways (often, only in relation to the wind) where executing a turn would be inadvisable.