Although his swing looks a little loopy Jim Furyk is a great example of a one plane swing, he takes the club outside coming back but his delivery position is exactly on one plane.
Getting full mobility in your wrist extensor is important because it allows your wrists to move properly in the backswing so you do not have an open face at the top of your backswing
Position two in the Eight Step Swing is the half way back position where the club is parallel to the ground and at this point the clubface must be pointing straight up square or slightly in, players who roll the face open at this point are dead in the golf swing