There are two types of resistance and braking mechanisms on indoor cycles - Magnetic Brakes and Friction Brakes. This section deals with bikes that have magnetic resistance.
As the name suggests, a magnetic cycle uses magnets to control the level of resistance when pedalling. Quite simply, the closer the magnet to the flywheel the more difficult it is to pedal. When the magnet is at it's closest position to the flywheel it's almost impossible to pedal at all!!
The resistance level is controlled by a lever or knob on the frame of the bike that you can move when cycling - so if you feel like simulating a hill climb part way through your exercise programme you move the level or turn the knob (depending on the model) to move the magnet which increases the resistance. Now you feel like you're cycling uphill! The reverse is true - reduce the resistance by moving the magnet away from the flywheel and it's much easier to pedal.
The main advantage of the magnetic resistance bikes is that there are no friction pads to wear out and therefore maintenance is reduced.