Another picture of it can be found at Murrays catalog. http://www.murrays.com/archive/29.pdf
There are different variations and block styles (as you can see), but the operation is basically the same.
...and yes, they do move the Jib inboard or outboard to close or open the slot between the main and the jib. This adjustment helps keep air accelerating across the back side of the main, creating more lift for the sail.
To use them, the jib traveller setting (the car in the track) should be approximately the same distance out (percentage wise) as the main traveller. Technically, every time you adjust the main traveller (usually on a tack or if you trying to power up or power down), you should adjust the jib traveller.
One tip : Never have the jib in MORE than the main, this closes the slot between the main and the jib, inhibits airflow behind the main. Result - you slow down. So if you are sailing solo or don't want to mess with it, keep it outboard, then just set it and forget it!