AWD is a reactive system, and can still be part time. AWD vehicles sense which tire is in a low traction situation and transferres power to the ones that have more traction. In the case of my VUE, it is a FWD car untill one or both of the front tires lose traction. The system then comes online and transfers power to the rear. A 4wd system is typically a manually activated system either via a switch or lever. Some of the simpler vehicles will also require you to get out and manually lock the front hubs in order to work correctly. These part time systems cannot be used on hard surfaces as they need to be able to scrub off differences in wheel speed on turns. Using them on hard surfaces will result in a condition called "Axle windup" which can do severe damage to the drivetrain.