Andrew- Basics of traction control (and this will include slip angles :-

Contrary to popular beleif, a robot is not generating the most force if its wheels are spinning- the most traction from a wheel is obtained when the wheel is only just on the verge of slipping. So if you want to most traction when pushing another machine, you want to reduce the power of the motor (assuming the wheels have been slipping) untill the wheels are only just about to spin.

A slip angle is the angle at which the surface of the wheel will just break contact with the surface that it is on. The slip angle therefore depends on the surface youre running on, the amount of pressure on the wheel etc. For most traction, you must apply power to your wheels so that they only just begin to slip- this is the slip angle. I invite james to explain this far better than I because Im not very good at explaining such concepts, and hes had a lot more experiance with this kind of thing that I have.