Howstuffworks is fine. This is a very simple 4WD system. Even those with the auto hubs or ADD flanges essentially work the same way.

Auto hubs, like manual hubs lock the front wheels to the front axles. Auto hubs are failure prone and were not common. If you have them it's not a bad idea to swap to manual hubs. When you shift the transfer case in to 4WD it simply engages the front drive shaft. Now the front and rear drive shafts spin together and this is why you should not drive on dry pavement. Snowy pavement is another story, 4WD is fine.

The ADD system has flanges that lock the front wheels to the axles all of the time. However, there is this little sleeve that disconnects the d-side axle from the differential internally when in 2wd. The shifter again just engages the front driveshaft. When in 2wd, the halfshafts spin bu the driveshaft and ring gear to not.

The ADD system can be shifted on the fly. The manual hub system can be too as long as the hubs are engaged. They auto hub version cannot be shifted on the fly if I remember correctly. You can shift on the fly at most any speed as long as you aren't spinning the rear tires.

You need to stop to switch into 4LO. I have been told my auto can shift into 4LO on the fly but I haven't tried yet.

Frank