Hi Nancy, are you sure we are talking about the same kind of knot? This is an embroiderers knot and the reason for doing it is so that there is no knot showing through the back of the work as embroidery is usually done on one layer of fabric. I think the reason for starting from the top is that it means that you are starting your stitch from exactly where you did the loop so there is no knot and there are no gaps. It makes for a very smooth start. I have taken some photos that I hope will explain the process better:
1. Thread the needle with the two loose strands going through the eye of the needle
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2.Leave a loop of about 1" and come back up through the fabric one thread away from the loop
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3.Thread the needle through the loop and pull tight
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So on the back there is a barely visible stitch. And you are ready to start stitching on the top.