Not sure where I learned of this use of fusible thread in the lower looper and checked in this tip section thinking I read it here but did not find it. Thus decided to post my recent experience with this.
I made the Santa Monica Tee today and decided to follow the instructions with the pattern of serging the edge then turning under. My past experience with this has not always been very good. Sometimes the knit stays in place well enough to topstitch or even coverstitch. Most times not. I normally use Steam a Seam but that can be a pain sometime to try to make a straight tape bend around a curve and stay until you iron/fuse it.
I have had Fusible Thread for awhile so decided to use in the bottom looper of the serger and serge finish the neck, sleeve and hem of the top as instructed. I then turned under and pinned the hem. I then fused with the iron. It worked great and held the fabric well enough to topstitch it.
I think this will work with other fabrics as well as a nice way to fuse in place before top or coverstitching. If experienced enough in turning and pressing, you can probably eliminate the pinning part.