Understitching?

 Despite a 30-year hiatus from garment construction, the dress I'm working on looks good so far.  But Step 14 was new to me!  To "understitch," the instructions say,"Press facing away from garment; press seam toward facing.  Facing side up, stitch close to seam through facing and seam allowance."  Come again?

In this picture, the upper side of the strap has not yet been understitched.  The lower side has been understitched and turned - to make a very soft, pretty edge.  If any experienced dressmakers read this post, can you please suggest what sort of interfacing is used for dresses these days?  I had fusible web on hand, but it created too much bulk.  Once I figured the understitching thing out, I still had some trouble wrestling the interfacings into submission.  Also, this pattern does not come with a sleeveless option.  I had trouble fitting my homemade interfacing pattern to the arm hole.  Seems like it was easier when I made the mock-up dress out of muslin.  But this slippery, solid fabric is harder to work with and shows every little flaw.  For instance, in this picture it looks like the right shoulder strap is askew, but when I examined it, I realized the dress will lay just right when there is a 3-D person wearing it, rather than a 2-D hanger.

Assuming the dress fits Miss Kami properly, I'll be making it one more time, in floor length, in Ivory Peach, as part of a Zelda costume that will look something like this:

I am to be in charge of the ivory and purple portion of the dress.  Kami will be in charge of the rest.  Will have to report later on how this project progresses!  :)