Alex Anderson showed cutting every third stitch on the bobin side with a seam ripper and then pulling on the top thread. It works well and doesn't stretch the fabric. It is also possible to use a "mustasch" trimmer to gently touch the stitches in a held open seam. It takes a little practice but works well especially on long seams. The trick is to hold the shaver with wrong side down. The opposite way you would use it as a shaver.