Driving offroad applies a lot more pressure and force on the steering wheel. When you’re bouncing over rocks or following the ruts in sand or mud, the steering wheel will often pull or jerk side to side. It’s important not to place your thumbs inside the steering wheel.
When you’re offroad, the tyres will find themselves bouncing over all kinds of ruts, potholes, rocks, and other objects. This can cause the steering wheel to suddenly turn. A sudden jerk of the steering wheel will cause the spokes of the steering wheel to spin around resulting in your thumbs being bruised, dislocated or worse!
If you make it a habit to keep your thumbs safely outside the steering wheel where they are out of the way of the spokes then it should become second nature to you.
With today’s modern power steering systems this technique is not as important as the days of manual steering boxes and no steering stabilizers. Still, with more prevalent higher horsepower engines and rougher driving styles it is a good idea to think of your thumbs and save them from the pain and bruising.
In an accident situation on or off the road, this technique is also good driving practice as it saves your thumbs as you are jerked forward. With thumbs on the outside of the steering wheel, they aren't bent backward.