The Downy fabric-softener ball has to be one of the simplest inventions ever. The ball is a self-contained timing device, meaning that you don't have to wait for the spin cycle to insert the liquid fabric softener into a load of laundry.
The ball utilizes Newton's first law of motion -- "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force" -- in an ingenious way to dispense the liquid at the correct time during the wash.
Initially, the ball floats on top of the water in a top-loading washer (front-loading washers cannot use the ball because it needs a vertically aligned cylinder for the release mechanism to work). The ball stays this way through the first few normal cycles. The liquid fabric softener is kept inside the ball by a rubber gasket connected to a rubber weight that hangs toward the center of the ball. The seal that is formed can withstand a fairly hefty force, which you can see for yourself if you try to open it.
As the spin cycle begins, the ball is thrown against the side of the washer with a great deal of force. This is where Newton's first law comes into play.
As the ball travels towards the side of the washer, both the hard outer shell and the rubber weight in the center are travelling at a certain speed and in a certain direction -- they have momentum. Once the ball hits the wall, the outer shell is immediately stopped, but the rubber weight can still move inside the ball. The weight keeps travelling with the same momentum toward the wall. Since the weight is connected to the gasket, once the weight reaches a certain angle the gasket is popped and the seal is compromised. At this point, the ball sinks, and the water in the base of the washer flows in to mix with the fabric softener. The mixture then flows out of the ball and into your clothing at precisely the right moment -- and you didn't have to time it yourself!