Originally Posted by Dirtbike

Its something that is hard to understand at first, thats hard to explain in typing, and is hard to explain without getting too in depth. I really wish there was a Flash movie explaining it or something.

Here are bikes with a Floating brake arm installed. Note that the caliper is not mounted on the swingarm or suspension, but rather the brake arm.
http://braketherapy.com/floatmodel.htm

Brake jack or chatter is when rear suspension acts differently when you apply the rear brake. The suspension will skip on stutter bumps when under braking instead of tracking over them. On a bike with a floating brake, the brake caliper's position relative to the ground is not influenced by the position in the travel the rear suspension is at. The caliper's position stays the same from the beginning to the end of the travel. The braking forces remain seperate from the rear suspension... it goes to the frame. When the caliper is attached to the swingarm on a single pivot, (no floating brake) the forces go to the swingarm, making the swingarm want to turn with the wheel.

Hopefully that gives you a vague idea of brake jack and brake forces. I need to get some sleep.