"Barking" the turbo is technically stalling the compressor of the turbo or compressor surge. I'm not realy knowledgable on the subject, but I think it has something to do with drive pressure being lower than manifold pressure, and the air actually moves in reverse in the manifold, charge air cooler, and turbo charger causing the noise.
I've heard several different stories on whenther or not it is bad for the turbo - And you could find more information on this on the Holset web site.