Regarding the action of the magnetic braking system....
The magnets are permanent magnets, therefore there is no variation in the strength of the magnetic field. There is, however, a variation in the strength of the force (meaning the braking force) generated by this field as the force is generated by the electric current which is generated in the brake fin as the fin passes through the magnetic field.
One of the experiments you should do in a Physics or general Science class involves a bar magnet and a coil of wire. What you learn from that experiment is that when you pass a conductor through a magnetic field, a current is induced in the conductor. In inducing that current, the moving conductor is subjected to a reaction force, and this reaction force is what Intamin exploits with the magnetic brakes. In general, the faster the conductor (the brake fin) moves through the magnetic field (the brake), the larger the induced current will be, and therefore the larger the reaction force will be. The upshot of all this is that the braking force of the magnetic brake is directly proportional to the vehicle speed! The faster the train is moving, the harder the brakes will hit, so the same length of brake will stop the train regardless of the train's speed.
Neat, huh? :)
--Dave Althoff, Jr.