My mom used to use a single edged razor blade, pulling the fabric taught to expose the stitches as much as possible and just tapping them. She could do that very fast. It would not be a good idea to try it on small stitches or on any quilting stitches where you can't pull the layers apart. My mom was not a quilter, but an excellent dressmaker. You don't want to do it that way if there is anyone around who might startle you. I cut myself just a little one time and went back to old fashioned seam rippers. In one of Eleanor Burns' videos someone is shown cutting stitches with a rotary cutter. I don't remember if it was EB herself or a student in one of her classes. Seeing that gave me the heebyjeebies! Even if you manage not to amputate a finger, the odds seem rather high that you could destroy your fabric.