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.
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
Many slicers hit the ball to the right simply because they are lined up to the right, when they go to hit the ball their mind knows the target is to the left and then subconsciously changes the swing to outside in so that the club travels towards the target causing a slice