Physics of robot base vs. width

[Vincent R]

One of the basic techniques to improve 6 wheels arcade drive is to lower the middle wheels to allow the robot to "rock." I understand that this is to effectively reduce the robot base (distance from back wheel to front wheel) and improve turning.

My question is how do you explain this phenomenon using physics? The way I think of it is that force is applied by the wheels producing a torque about (assuming) the centre of all the wheels. But whether if it is 4 wheels or 6 wheels in contact, the perpendicular distance from the force applied by the wheels to the axis of rotation remains the same and the centre of mass stays relatively the same when you are "rocking". So how does this improve turning?