Scaling is precipitation on a surface of the pool or on an SWG plate.
Where there is more magnesium in the water, then the scale may incorporate some of the magnesium into it so instead of forming calcium carbonate "calcite", it forms some calcium magnesium carbonate "dolomite". Magnesium carbonate itself is roughly 10 times more soluble than calcium carbonate which is why we usually only concern ourselves with calcium carbonate with regard to scale.
If Mg can help prevent precipitation they why don't more people add it to their pools? What's the downside?
I understand I'm mixing 2 different areas here (nature and swimming pools) but I would think some of the same concepts apply.
Here is an excerpt from an article written by Randy Holmes Farley, a well known chemist in the reef world:
http://www.advancedaquarist.com/2003/10/chemistry
How does magnesium interfere with precipitation of CaCO3? The primary way involves magnesium poisoning the surface of growing CaCO3 crystals, slowing the precipitation. It can, in fact, be slowed to the point where it simply does not happen at rates problematic to an aquarist. In the following discussion it is important to remember that, other things being equal, alkalinity is a good indicator of the concentration of carbonate. So higher alkalinity equates to higher carbonate.
In short, while magnesium carbonate is not supersaturated in seawater (or in typical reef aquaria), and will not precipitate on its own, magnesium is attracted to calcium carbonate surfaces where the carbonate ions are already held in place by the calcium ions. With the carbonate ions held in place, magnesium finds this an attractive place to bind.
After a short time in seawater, a virgin calcium carbonate surface quickly attains a thin coating of Mg/CaCO3 (magnesian calcite) as magnesium pushes its way into and onto the crystal surface. Eventually, the surface contains a substantial amount of magnesium. The extent to which this happens depends on the underlying mineral, and is apparently much more extensive on calcite than aragonite. It also depends upon the relative amounts of calcium and magnesium in the water. Regardless, a new type of material is formed that contains both calcium and magnesium.