How do mechanical sewing machines produce zig zag, satin, feather, and hundreds of other stitches?
A single stitch is formed when thread from the upper tread spool flows between the tension discs, take up lever, and eye of the needle. The needle moves down piercing the fabric and drawing the upper thread beneath the needle plate. As the needle rises, the point of the hook moves behind the needle picking up the upper thread and drawing it around the bobbin thread. The needle continues to rise pulling the threads taught as they form a knot inside the fabric.
But how do you get zig zag stitches?
The stitch selection knob on your machine has a lever that reaches inside to a tracker or follower lever that rides on the outer edge of a cam gear. This edge has ridges and valleys on it. As the follower rubs against the cam gear edge, it moves in and out along the ridges and valleys. The follower is also attached to a long lever that reaches across the machine to the needle bar assembly.
As the cam turns, the lever to needle bar moves in and out. On one stitch, the needle bar may be in the far left position, and on the next on the far right position controlled by the lever coming from the cam follower.
Vary the stitch length, stitch width, and the forward or reverse stitching, and you get all those many different stitches all controlled by the bumpy edges of the cam gear.