In the spin cycle the ball is forced against the wall of the tub. There's a little weight in the center of the ball that's attached to the rubber seal. The outside of the ball is against the tub so it can't continue to move outward but the weight does continue to move toward the wall, because damnit, Newton said so. This breaks the rubber seal. When the spin cycle ends the ball drops to the base of the tub and the water mixes with the fabric softener.