Probably when I-cord was first invented, two generations of knitters thereafter worked on figuring out how to keep it from spiraling! Then, oh happy day, it finally lay flat!!!
And now here we come...... oh, well! It might not work but here goes...
Work with a fairly long piece of I cord. Thread a blunt-end tapestry needle with yarn. Pull some through the eye of the needle but leave the rest attached to the ball. Starting at the bottom of the I cord, work the needle up the length of the I-cord inside through the center. Keep working until you reach the top, pulling the yarn on the needle up with you. Pull the needle out the top, snip the needle's yarn off at the bottom (leaving enough to tie a bow). Once the bow is tied at the bottom, start pulling the thread out the top which will gather the I-cord. As it gathers, I think you'll be able to work it into a spiral shape.
If so, then secure the top end of the yarn and make sure the bottom end with the bow is also secured.
Please let me know if this works! Apologies to those long-ago knitters who invented I-cord, but I just had to try solving this neat knitting puzzle!
PS - Some sewing supply stores have these long metal things with a hook on the end made for turning tubes of fabric right side out. If you have one of these it might be helpful for going down through the center of the I-cord and snagging the scrap yarn. Even a long afghan hook would probably work. You can use the tapestry needle but you will have to fiddle it up the center.