Yes you are right, you can have the revs drop lower without stalling it, you usually take off in 2nd gear. But climbing steep hills or powering through mud, if you change gears too soon revs drop too low (kind of like the way it works in a petrol car) and it loses momentum.
Driving a car like that is different, but most of the concepts are the same - ie recovery. So I am probably NOT right in respects to petrol 4wd and revs etc
Anyway I am sure Jason will learn what is right for his car when he does the course.