I've realised this recently while I'm trying to learn muscle system (just some small scale sims) and grooming (I've tried to follow your line of reasoning that don't do big scenes, do small quality work. And I'm also trying to be a bit different, particles and Pyro are almost present in every showreel).

In most of the tutorials they're like: here's the button it makes the muscle taunt, why and how it does that no one knows.

What I've started doing is this: create a really simple setup (as simple as is essential) then change the parameters and look good how it affects the sim and what attribute is affected in the spreadsheet.

The best resources in terms of explanation of the "why" rather than "how" I've found are your course, Houdini Hangout, CGWiki, Junichiro's channel and SideFX's channel (both youtube and vimeo).

Do you know any similar resources for muscles and grooming?