+1 to whoever said, generally speaking, there is NO time to react...it just happens. Judo players, acrobats, etc fall in very predictable ways, out on a descent with a gravel corner you won't have time to react...it is what it is.

I think the best way to learn how to crash to put yourself out there and continue to do it (aka practice crashing) on a regular basis.

IMO it comes down to when to accept a crash vs when to try and ride it out. There is a decision point we all have to make and I think it simply comes down to how experienced of a rider you are. Trying to ride something out, which I see all too often, is a recipe for disaster....as it bailing too early.

I also don't think "tuck and roll" is a great thing to ingrain into muscle memory, landing on the back of my neck / spine on a rocky descent is the last think I'd want.