Who else is still going? (Also, thank you to whoever it was that said most people fall out soon after Brachot. You gave me the push to get through Demai.)
I have an explanation of Rabbi Yehuda’s position in the last mishna of this masechet, partly based on Bartenura and partly on my own practical knowledge. Bartenura says that one who passes a needle through two layers of fabric twice (i.e. one full stitch, with both ends of the thread now on the same side) is only liable (for kilayim, or undoing a stitch if he does it on Shabbat) if he knots the two ends together, because if he doesn’t it’s not קיים .
Try this yourself if you have a needle, a thread, and a scrap of fabric: thread the needle, put it through the fabric at point A, and back through at point B 0.5cm (1/8″) away from point A. Now, pull on one end of the thread. Unless something got knotted up, the thread will pull out as easily as a slipknot or bow is untied (a more familiar action, generally allowed on Shabbat.) This is why Bartenura says the two ends have to be knotted for it to count as קיים.
So now for Rabbi Yehuda’s position : thread the needle, sew through points A and B as before. Rather than continuing in the same direction with the third stitch, pull the needle through at or very close to point A. Now try pulling on one of the thread ends. It shouldn’t pull out easily. Congratulations! You’ve sewn a full backstitch, one of the best hand sewing stitches for durable seams and commonly used historically. No knots needed.
Chodesh tov and thank you for coming to my Tedtalk.