The accelerated action refers to the balance rail which is rounded, not flat. It has no reference to any actual action part, like the whippen. So the keys pivot on a rounded surface instead of a flat one.
In my opinion, on a mechanical technical aspect, this is a beneficial development as it allows the keys to pivot on a much smaller area which allows for a faster shift in weight which means faster keys response.
And since Kawai Millenium 3 action was mentioned, it is far superior to any current manufactured whippen. It is significantly stronger and stiffer and thus has a very fast response. And from a maintenance perspective, it will outlast all wood-made whippens since it is significantly less affected by climatic changes.