The feed dogs are the mechanism which moves the fabric forward as the sewing machine creates each stitch. They are a pair or series of narrow metal plates beneath the presser foot which are equipped with teeth to grasp the fabric. They are driven by either the lower or upper shafts, and controlled by two linkages known as the feed carriage. One linkage moves the feed dogs forward and backward with each cycle. A second linkage moves the feed dogs up and down. The feed carriage is synchronized with the needle bar, so that the feed dogs press up against the fabric, move it forward, and then lower, releasing the fabric. While in the lowered position, the feed dogs then move backward and repeat the cycle.
The range of travel of the feed dogs is controlled by the stitch length regulator, a lever or knob which determines the length of each stitch.
Reverse - On sewing machines equipped with a reverse function, the feed dogs reverse direction, enabling the operator to stitch in the reverse direction. see Back Tack