I ride my track bicycle on rollers for training purposes. While rollers give a very realistic riding experience, and are good for pedaling technique, they don't provide enough resistance for building muscle power. One ends up maxing out leg speed (this happens for me at 160-170 rpm) before running out of strength.
Mounting a magnet very close (2mm or less) to the aluminium roller surface generates eddy currents in the roller when it spins. The eddy currents generate an electric field that interacts with the block magnet's magnetic field to generate a force that acts to slow the rollers down, converting the rider's effort into heat due to electrical resistance to the eddy current. The force provided increases linearly with the roller speed, so the faster one rides, the more resistance one feels.
The three Neodymium block magnets I purchased from AMF Magnetics currently provide my rollers with over 900W of resistance @ 150rpm which is all my current fitness and strength can handle. With consistent training, I hope to need another magnet by the end of the year.