The tension spring is a little wire the thread hooks in as it passes through the tension then on to the take up lever. It travels just a bit as the thread is pulled through by the needle. I had a Spartan that had a messed up tension spring. I had to unscrew the tension at the side of the machine and do some adjusting to get it to hold in place. I found that info on the sewing machine repair website listed in the link I posted earlier for tension problems.