If your CR-V is from 2011 or earlier, you have a hydraulic system, meaning that there are two different pumps for the front and rear wheels. If the two pumps become unsynced, the difference between them activates a multi-disc clutch that splits the torque between the front and rear wheels.
If your CR-V is from 2012 or newer, you have a computer monitor that switches your car into 4WD if it detects any slipping.