He was referring to the fact that when you stab the breaks, the center diff will only apply stopping force to the wheels that are not griping the surface at that point in time (well which ever wheels have less pressure on the racing surface) which will defy the point of having breaks. lol So with a spool, it will allow you to break with all for wheels which will be a good thing for when you need to stop the at high speeds.

Also maybe you could move the motor towards the center of the car (front to back) to help make the car more balanced.