As I ease into my retirement from ultras and trail races, I thought it'd be a good idea to say what I learned over the past 4 years that I wish I'd known when starting. The next few posts will be about things I've discovered, that most people learn by experience and never think about.
The first time I ran "technical" trail, I wondered whether it was better to run between all the rocks and tree roots or to try to run over the tops of them. Watching those in front of me didn't give me any answers. The answer turns out to be that neither is correct; when you're experienced at technical trail, you keep your stride length as constant as you can and step wherever that puts your feet. First, you learn that tree roots rarely move (I remember vividly one that crumbled into dust beneath me) and that large rocks don't move much, though smaller ones can shift - scree fields are a completely different world, not seen here in the midwest - so, running on top of obstacles is not that difficult. Beginners try to step between all the obstacles, because running on the bit of solid ground between obstacles is familiar, but this means making odd steps in different directions, throwing off one's balance and tiring out some smaller muscles. The constant shifting in stride length is much more tiring than just trying to run straight through. Until you get used to learning to balance on uneven surfaces while running, this is difficult and often leads to twisted ankles, sore knees and occasional falls, but after hundreds of hours of running trails, one's muscles strengthen to stabilize joints during the moments that you're not solidly planted on the ground.
Muddy courses also give beginners pause. The common idea is to try to avoid as much mud as possible, by skirting around the edges of puddles. The biggest problem with this is that most courses are either loops or out-and-back, so even if you're leading the race, at some point you will hit the same mud holes after the entire area has been churned by hundreds of runners and all relatively dry areas are gone. Doing training runs in the rain, especially on trails, teaches you that getting your feet wet is not the end of the world; eventually you get to the point that you aren't getting blisters from running in wet shoes (either from finding lubricants that work for you or, more commonly, from just having your skin toughen). The best method to handle mud is the same as for rocks and for everything else: keep going as if it's flat, even and dry. If you do try to stay dry as long as possible, the best plan is to step where you can see footprints that aren't too deep; if it was okay for the runners ahead of you, it's probably okay for you.
The one remarkable difference I've seen between frontrunners and back-of-the-pack runners on technical courses is that the faster runers keep their eyes focused ahead, while the slower ones are looking to see where to put their feet. The faster runners scan ahead for obstacles and seem to float over them effortlessly, whereas the slower ones are looking down, dealing with immediate issues. When you're confident in your ability to handle technical trails, you just simply run them without thinking about it, and that's why no one ever seems to tell you how to run them; they've learned it subconsciously.