I think Kev is right, they are trying to not fight. They don't want to do what they are doing
Anakin has stormed the Jedi Temple, slaughtered those in it, traveled to Mustafar, killed every Seperatist leader, and Force-choked Padme. Why wouldn't he want to fight Obi-Wan?
Someone made a post over at MF (actually a cross-post from CHUD) that made a lot of sense, mentioning that the move was something that experienced swordsmen often do, a sort of "feeling out" of the opponent, especially when they're too close to come to blows. They spin their swords, learning how the opponent will attempt to counter, and find a move to come out of the spin with to attack. Well, something like that. Made sense when I read it, and I'll stick to that until I see the scene in context.