What do you think this means?
Normally you strap the parachute to your body, then you strap yourself in the aircraft. Egress after a normal landing is the reverse - you undo the seat belts, step out of the aircraft and only then undo the parachute. This procedure is obviously trained this way so that if you're ever in a hurry to leave the aircraft (after a midair collision causing structural damage, say), you *only* undo the seat belts, so you leave the aircraft together with the parachute. Ingrained habit, that sort of thing.
In a ditching scenario however, the parachute becomes a liability instead of an asset. After all, it's a bag filled with a load of fabric, and will retain a significant amount of water once it's been immersed. So it's better to leave it in the airplane. So you have to undo the parachute straps before exiting. And since you will be in a hurry to exit once you're in the water, it's best to undo the straps before you ditch, not after.
That's the theory behind it, I think. Whether practically speaking this is going to work remains to be seen. My gut feeling tells me the seat belts will get in the way of undoing the parachute straps.