Bumpsteer happens when the steering liink ( the rod between the spindle and the steering rack ) follows a different arc from the lower control arm.
Bumpsteer kits allow you to shim the outer pivot point of the tie rod up or down to get it to follow the same arc as the a arm.
With mustangs you also usually have to modify the rack mount bushings as well to get everything lined up.
Bumpsteer is really a major problem under heavy braking on uneven surfaces. It causes the car to dart around like crazy. | 
