In 2WD the front driveshaft and axle are disengaged. With A4WD the front driveshaft is engaged but the front axle is not. If rear wheel spin is detected, the front axle is engaged and you now have temporary 4WD. After a period of time, the front axle is disengaged once again but the front driveshaft remains engaged. In straight 4WD (hi or low) both the front driveshaft and axle are engaged full time.