What keeps the chains from falling off the rack while you are driving down the road?
The specific angle of the rack and stop plate keeps the chains from being tangled or falling off. They all stay in their specific place until you need to chain up.
Does the chain on the end keep the chains from falling off?
No. The chain on the end is intended as an anti-theft device that can be locked. It is not necessary to have the chain locked while driving. This saves time. Lock them up when leaving your truck.
What if I don't have room on the other side of my truck for another rack?
One rack will hold all the chains you need for all 8 drive tires. It is more efficient to have a rack on each side, which keeps you from having to drag your chains from one side of the truck to the other.