Ahhhh…you asked the right person. 
The reason a jig works is that the relationships between the elements of a design are fixed, when they are created in the same file.
So when you create a cut line that is the size of the cutting board, and then align the artwork inside of that cut line, the relationship between the two is always maintained perfectly.
Then you just cut the rectangle, take out the center area, drop in the cutting board which is the same size, and the placement of the engraving is going to land exactly where it needs to be on the cutting board. Then you run the engrave.
There’s a brief writeup on it here:
The reason that is necessary, is because of the fisheye effect of the lid camera. The view is always going to appear to be farther away from the actual engraved result at the left and right edges of the bed. They are farther away from the lid camera at the center.
There is an algorithm that compensates for that effect, but it is dependent on the thickness of the material to calculate correctly and adjust the view, so that is why we are so concerned with getting the thickness of the material correct for the calculations. It adjusts the view we see on the screen.
Recently, Glowforge released two things…the ReCalibrator and the Set Focus tool. Those do all of the heavy lifting now, and will correct the distortion, but you have to run the ReCalibrator and use the Set Focus tool before the print to adjust the view near where you are wanting to print in order to get corrected results.
These tools are discussed here…if you haven’t run them yet, you should do that…it improves the image you see, making visual placement much more accurate.
You might be able to get by without a jig if you run them. (And it’s getting to be past my bedtime, so I’ll check tomorrow if you have any other questions about it.) 