Basically these are a wind trainer with very low inertia and a big fan. By having a very low inertia means that you must drive the pedals all the way around. It also means it is impossible to get on top of the gear, making it a nasty nasty piece of equipment to do all out sprints on.
The main difference between the BT Erg and a standard mag (magnetic) trainer is that a mag trainer only applies a static resistance. This means that the power required to drive it at one speed is a direct ratio to the speed. So if you double the speed you’ll need to produce twice the amount of power. The BT Erg is similar to a fluid trainer as the resistance is dynamic. This means that the power required to drive it is not a direct ratio of its speed. It has some expontenial factor to it, You need more than double the power to make it go twice as fast. This can be seen in the following graph.

The difference between the BT Erg and a Fluid Trainer is that the BT Erg does not have as much inertia. The Fluid Trainer will have a flywheel to make it feel much more like you are riding on the road. Even though the flywheel on a fluid trainer is small it will be moving at a high speed giving it more inertia. The BT Erg is a big fan that is light and moves relatively slowly compared to the flywheel of a mag or fluid trainer. Taking away the flywheel means you need to keep up the force to the pedal stroke all the way around to keep it moving.

If you are training with a BT Erg you’ll find a graph of the Power Curve and a Table and Equation to help with calculating the gear and cadance to use to find the desired power output useful.

The benefits of developing the whole pedal stroke is very useful. It means you can apply more force to more of the pedal stroke resulting in the production of more power. At sub-maximal efforts you end up using a small proportion of your reserve, meaning you can hold off muscular fatigue for longer. By having the low inertia of the flywheel and drive train it forced the rider to recruit every muscle that can be used in a pedal stroke, this improves the motor program used for pedaling that the rider will then subconsciously take with them to riding their bike on the road or track. Secondly this recruitment of the muscles will provide a stimulus on them, particularly those that are not usually recruited, and they will develop to improve performance.

From Hamish Norton’s blog