Some sewing machines take double layer cams and the stitch patterns have a automatic reverse movement in the stitching. It allows for more advanced stitch patterns.
Stretchy stitches generally mean utility patterns that give a seam with a noticeable stretch, more so than regular zigzag and mostly used for jersey knits and fine knits cuffs and similar. The paterns aren't exactly what you get on a serger or cover stitcher (which has two more needles and loopers for their equivalent of "bobbin" thread). These regular machine patterns are good and work very well though.
In addition these cams can give patterns with extra strength, fancy decorative patters, etc. Elna Supermatics had this as early as 1952, and a few other brands quickly followed up with the same feature on their models; Necchi Supernovas, Adler, Gritzner, Kayser. Husqvarna didn't have them until the early 1960s, Singer not until the 1970s. Here's a picture of Singers Flexi cams showing the stitch pattern.
This is Elna cams, the red patterns are double layer.
