From my personal experiance I leave it in Auto while driving. If you get in a situation where you have low traction (sitting on ice/slush/snow) and need to get out then just put it in "lock" mode (dial all the way forward and hit the switch next to it).
Also I sure hope you arent on the stock tires and trying to drive in bad weather.
The DCCD is one of those nice options that IMHO could have been excluded. Auto has treated me fine in the dry and once I get going in the wet stuff. Lock does a great job of plowing through stuff.