The Deceleration property gives you the facility to turn a linear animation into a non linear one.
In a similar way to the AccelerationRatio property, you specify the property and the animation makes a gradual transition.
In the case of the Deceleration property, the animation gradually slows down.
In the following example, the bottom ellipse is slightly out of the line with the linear animation in the top ellipse.
This is because it is using a gradual slowing down (or deceleration):