Jean Genie has many uses.  The first and most important is that all you need to do is place the Genie over the jeans and mark the hem allowance.  Obviously there is a little more to that and you will see what I mean by watching the video.  So why did I create it for this purpose?  I can't tell you the amount of times a person came to me with jeans that had been cut too short.  And this usually happened because they forgot about the seam allowance and cut on the new length mark.  
So that is the first reason.  The second is that jeans have a very thick seam, and the Genie allows you to step over the seam.

In the video I also explain how to sew what I call "topside".  This means you will create a professional jean hem with two threads threaded through the machine and one thread in the bobbin.  Check out the video.  Keep this url link, because you may want to come back and view the video a few times.  Practice makes perfect.