The first time I ever drove in any serious off-road terrain, my friend Andreas told me to keep my thumbs straight and outside the steering wheel, rather than hooking them inside. He pointed out that in rough terrain, where the front wheels might be forced quickly to one side or the other, the steering wheel will spin rapidly to one side or the other, and possibly hurt your thumbs badly in doing so. I've kinda made a half-hearted effort to do this all the time, but never taken it seriously. Hooking them inside is a little more natural and comfortable, and allows a crush grip on the steering wheel. On the other hand, I probably don't need a crush grip on the steering wheel.
During the InSights Street and Vehicle Tactics course, Greg Hamilton pointed out that the same rules apply while wrecking a car. Additionally, it's natural for us to tense up our arms (and legs) in anticipation of a crash (pushing our body back from the steering wheel and floorboard). When the vehicle slams into something, your body's inertia is going to bring a lot of force to bear again just your thumbs, if they're hooked inside the steering wheel. He cited a couple examples of serious, life-long hand injuries sustained in this way.
So I'm going to work a little harder to keep two thumbs up, rather than hooked inside the steering wheel.