The transfer case for driving the rear wheels mounts directly to the back of the transaxle case and is part of the transaxle. It provides a direct drive (1:1 gear ratio) coupling to the rear differential. This means that when the 4WD unit is engaged, the transaxle provides equal power to the front and rear differentials. When the 4WD unit is not engaged power is transmitted to only the front wheels. In either case shifting of the transaxle remains the same. Late models have dual range 4WD.
The drive selector can be shifted at any time, with or without clutching. However, if you shift the drive selector while the car is moving the steering wheel should be in the straight forward position. This minimizes the load on the rear drive system and shifting is made easier.
You may feel a braking action when turning a sharp corner in four wheel drive. This is a normal phenomenon which arises from the difference in turning radius between the front wheels and the rear wheels, and will not occur when running in front wheel drive.