I learned about this in a class from Viking educator, Alix Graham-Michel. Most serger rolled hems are with one needle. This is with two needles, both the left and right needle left in. You set up the serger as usual for a rolled hem but use both needles and 4 threads. It works! She said she discovered this because she was too lazy to take out the left needle. I think it gives a very strudy rolled hem and in this picture I used it on fleece.