Plasma is absolutely right about the inhumi being reptilian and being able to hibernate or shut down. But what is new about that---we've pretty much known that all along, almost since we met Quetzal (well, we didn't know about the hibernation). And I certainly don't think that's **the** secret. Mind you, I don't know what the secret is, but I'll lay money that that isn't it. Consider: Horn has sworn up and down to Krait, whom he loves as a son, that he will not reveal the secret. But quite a few helpers, gardeners etc. are in on the hibernation shtick.
I have to say that the theory of Horn's double vision wrt arms and legs seems exceeding far-fetched. Esp. as Seawrack has only one of the former and two of the latter.
-alga
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.