Complex sentence: Waterstarting is faster and less tiring but requires enough wind (about 14 knots or more) to fly the sail and get mast as vertical as possible, so a sailor could pull up against board's buoyancy.
Split into simpler sentences: Waterstarting is faster and less tiring but requires enough wind to fly the sail and get mast as vertical as possible (about 14 knots or more). The key to waterstart is to help wind to get mast as vertical as possible, and then pull up against board's buoyancy.