@spectra
its easy... just like in the sports cylcles the front gears work
high gear has a transfer ratio of say 1.0... when u shift to low the ratio is changed to say 1.8... so more engine rev, higher torque to the wheels with lesser top speed 
normally we drive in 2H...
when we lose traction we shift to 4H.. all four wheels engaged but no change in ratio
when stuck, stalling on steep climbs, we can shift to 4L for maximum torque.. engine rev on rated RPM for max torque... all wheels engaged with a higher ratio
the willys and the toyota jeep have the best low ratios to my knowledge.. they literally crawl when in low... in rocky i.e. my jeep's gear there is only a slight difference
thats the theory..
practicals on this sunday... 