Here is one quick post coming from the Microsoft Expression blog on fluid transitions between states.
Layout properties are typically set to discrete values (such as row or column numbers) or set to automatic values (such as width and height). To create a smooth transition between changes to these kinds of properties, use Fluid Layout. For example, if an object appears in the lower-left cell of a Grid layout panel in one state, but appears in the lower-right cell of that layout panel in a different state, the transition between those two states will result in an instant change unless you turn on Fluid Layout for the state group and increase the transition duration.