I am deadly serious about this. This is really a VERY BAD method of tying things down.
When you have a line stretched between two points, applying any pressure along the middle of that line puts a tremendous amount of inward pressure on the end points. Basic physics.
This is the same reason that power companies don't stretch power lines tight between poles. They sag for a reason. If power lines were stretched tightly, a sparrow landing in the middle of one and causing the line to sag minutely would be enough to snap both poles off at ground level.
This is a shorter distance, but applying 20 lbs of force correctly in the center of the chain, say toward the front of the truck, could cause enough inward force on both sides to bend in both sides of the bed. And if you used it to tie down something weighing more than 20 lbs, well, all I can say is, better not stop, start, or turn quickly.