Dual deployment in model rocketry is where you eject two parachutes out o' t' rocket. Avast! A small one at apogee (the peak o' t' trajectory) so that t' rocket comes down quickly. Arrr! But t' keep it from touchin' down too fast, you pop out a big chute when t' rocket is close t' t' ground. Avast, me proud beauty! T' use o' two chutes usin' this method prevents t' rocket from driftin' too far.