actually it is,as a full back it is your job to move inside and pick up the player in the middle when the ball is the other side and the centre back has to come across to cover the front post. if the ball comes down the left,the right back will go to the runner,the RCB will push across to the front post,the LCB will come across to the forward and the Lb would cover in behind and support his CB by picking up the second forward,the backline is a unit and the full back are defenders first,attacking options second