I have to look myself, it's not really what I spend most of my time in, but I heard a talk by Gottfried Schlaug from BIDMC in Boston and he made a few comments, may be useful to pubmed him and see if he's written any reviews or editorials.

tDCS is still an emerging tech, and I think not a lot of people have figured out how to do it. Transcranial magnetic stim has been around a lot longer. I don't think people really know the best way to do that either, but there's more experimental data showing that if you do it via certain methods/paradigms, you get a measurable experimental effect of some sort (usually a muscle jerk measured in the arm, or maybe performance on some standardized cognitive/motor skills test, etc).

The problem is that no one is exactly sure what it does. Most groups do not have an IRB to study this in tissue - getting animal IACUC approval is hard, and in humans, without a good medical reason to go through the cranium, forget about it... (i.e. most of the neurophysiology data we know today, aside from in animals, is from opportunities to record data in surgeries for epilepsy or DBS surgery of Parkinson's disease).

So for lack of a better term, the holy mantra of "neuroplasticity" is involved for a hand-wavy explanation of effect...