Thank you in advanced!
It acts as a four bar to keep the two nonrotating bars parallel. Think of the two length of chain as two arms of a four bar linkage.
The major benefit of a chainbar over a 4 bar is that you can only get 160-170 degrees of travel out of the 4 bar, and you can get well over 180 degrees out of a chain bar. The trade off is you lose some strength and add a little complexity.
If your limited on metal, a chain bar might be a better option too.
You can build a four bar to travel more than 180 degrees.
The chain is responsible for making the raising end parallel to the ground.
Yep, but not as compactly.