I'm probably the only person in the world who could pull this particular thread of thought together, so let me do so:

Rooksbridge and the Leashes harkens back to an old Josh Roby idea that (I believe) was published in Pyramid: the magical industrial revolution. Our real industrial revolution was a slow process of putting more and more power into more and more hands, via the arduous growth of a middle class and a whole lot of low-skill labor. The magical industrial revolution from Pyramid days involved lots and lots of people getting together to provide the mana to create magical things, which were then sold to this growing middle class. From the early Rooksbridge stories that looks a lot like what the leashes can do, too: they disperse power by democratizing it, creating a new class of leash-holders, who could eventually grow into being the middle class of their world.

Of course, it remains to be seen if leashes can do anything really useful, or just be used as fire whips. But if they can…