Concrete/Movable Ground is usually movable stuff that helps pigs to survive in levels. This version of it means it can move, but it still floats. Birds can push it, pigs can move themselves and the ground away from any threat to them, or sometimes after a bird attacks, pigs will move, with the ground, to a safer place.
Movable Ground is a type of material in Angry Birds (Like Wood, Stone, Snow, Glass). It cannot be destroyed, like original ground.
More info coming soon