My grandmother taught me this little trick years ago. But, since we did mostly home dec sewing then, she taught me while using dental tape instead of thread. I still make ruffles and gathers this way, and the dental tape never breaks part way through the gathering process. You simply tack stitch it at the beginning and then zig-zag stitch over the tape within the seam allowance. I've also started pinning the dental tape if its the top of curtains I'm gathering. I just un-do the pin and smooth out the top --- makes for easier laundering and pressing before re-hanging.