Complex sentence: The waistband is also held together at the front with a lace-up tie, in order to ensure that the shorts cannot be not pulled off by the force of the wave when the surfer is tumbled under water during a wipeout.
Split into simpler sentences: The waistband is also held together at the front with a lace-up tie. This double fail-safe system is in order to ensure that the shorts cannot be not pulled off the body by the force of the wave when a surfer is tumbled under water during a wipeout.