It works off the rule that if you charge an enemy unit then you turn to face whichever direction they are facing, they do not turn. So if you have the following line of 3 skinks facing a unit: xxxxx xxxxx xxxxx --------s ------s ----s --SSSSS --SSSSS --SSSSS And they declare a charge on you, which they are somewhat forced to since you are right in front of the unit blocking them from going anywhere else, when they hit you they turn their formation to line up with the front of the skinks. Now you can imagine saurus unit 'S' back behind the skinks suddenly has a perfect look at their flank to charge. Keep in mind this diagram is pretty crude and not to scale, you need to think about where unit x is going to end up when they overrun the skinks in order to setup a unit to hit their flank in as much of the possible area they could be in as you can. If you can get the unit right near the edge of unit x, you can also flee the charge and redirect them. I will let someone else explain that a bit more.