The engine turns the tires. So in order
Engine transfers rotation to Transmission, Transmission transfers rotation to transfercase(S) then through the drivelines into the ring and pinion gears which turn the axles that turn the tires.
Hard to explain but think of it as a math equation.
If I have a 3.93 to 1 1st gear ratio in my transmission, then a 2.28 to 1 low range in my transfercase and a 5.29 to 1 ring and pinion gear set in your axles then the way you determine your crawl ratio is
Transmission 1st gear times transfercase times ring and pinion gear = crawl ratio
3.93x2.28x5.29 = 47 to 1 crawl ratio
Stock dual transfercases double that ratio because your adding another 2.28 to the equation so instead it would look like this with the same setup in the example
3.93x2.28x2.28x5.29 = 108 to 1 crawl ratio
So by adding that second transfercase (crawl box) your increasing the amount of torque that is concentrated to those wheels, thus giving you better traction.
More gears = slower speed
I'm not very good at explaining but that gives you the rough idea, i'm sure someone will post up specifics.
