I recently started a job where I was introduced to HSM's. The answer is, there isn't one. The resulting C reads like the entire program was written in main() and connected via glorified gotos. Every module depends on a global event queue, and event data is passed via circular dependency on effectively global state. I have never seen such basic logic so obfuscated and so distributed.
It might just be this codebase, but I do not see the appeal to HSM's. I'll stick with my standard flow control (while, for, break, return) and compose functions from other functions. It sure is nice to make calls that actually return.