Yep Delmarrey is right. It isn't actually magnetics of course, but the particles when they clump together form a wax type material. Most liquids that are made up of multi-complex chemicals are subject to this. Although not quite the same, water and anti-freeze is a good analogy. The water molicles are effectively held apart by the glycol molicules, thus stopping the water molicules from collecting together and crystilizing. If you want to get really technical, the activity of the water molicules decreases as temperature drops. As the activity comes to a stop, they can collect together, and the only difference with water than any other liquid, is that they start forming a crystal. The crystal has an irregular surface and thus has gaps between on crystal and the next. Thus water expands when frozen. But that is off the track.
Diesel is made up of many complex chemicals. When it gets cold enough, one of those chemicals starts to "freeze" out of the others forming this waxy substance. The additive does the same job as the glycol in water. It's stops the molicules of that waxy substance from being able to come together and the wax stays at a molecular level floating through the diesel. At such a small particle size, it will still flow through the fuel filter
along with everything else that makes up the fuel.
What the additve is, varies from snake charmer to snake charmer. Some can be good, some can be dangerouse. I mentioned in another post that some of those additives can be Kerosene as the base carrier of the toxic stuff. Kerosene happens to be a good product for cleaning
injectors. So some of those products are sold as injector cleaner. But Kerosene is also death to the pump and injector, cause to those components, it acts like an abrasive instead of a lubricant.
Another product I have great doubt in, is the bio-mag filter. It may be called something different in your neck of the woods, but is is basiclly a magnet that the fuel runs through. It is supposed to pull apart the algea and the wax and stuff. Well I have yet to be convinced how it works. If a magnetic field could have an affect on the deisel components, it would also attract those components. And so far I have yet to see a tank of fuel sucked up by a magnet.
Oh and one final one out there. Anti-foaming agents. Often found in additives used to reduce heat in oils, such as hydraulics etc. It is basically a minute Silicon particle that has a "sharp" shape to it. The sharp points break the air bubbles down, thus reducing oil
temperature and foaming.