I'm pretty comfortable with this. It makes perfect sense to measure the changes in velocity over time of a known amount of mass to calculate HP. And its probably much easier than applying a constant resistive force, such as a brake or an electric motor functioning as a brake (generator mode).

I was under the impression that the mass type dyno's mask rapid fluctuations in the HP/TQ curves. I can't remember the exact name, but in the threads where a guy has the 'ultimate garage', he had a dyno system that attached directly to the hubs.