What are feed dogs and how do they work?

Feed dogs are the toothed metal bars that appear through slots in the needle plate. They move the fabric a set distance for each stitch, and they do it in a loop.

The motion. They rise above the plate, move backwards while gripping the fabric against the underside of the presser foot, drop below the plate, travel forward again out of contact, and rise. An oval. Crucially they only move the fabric while the needle is out of it — the timing of the feed against the needle is as important as the timing of the hook.

What follows from understanding this.

Stitch length is set by the feed, not by the needle. The length control changes how far the dogs travel on each cycle. Reverse works by reversing that travel.

Presser foot pressure matters, because the fabric is gripped between the foot and the dogs. Too little pressure and the dogs slip; too much and thick fabric is squashed and dragged.

Very thick, very slippery or multi-layered work defeats a single set of dogs, because they can only move the bottom layer. That is precisely the problem a walking foot solves.

Very fine fabric can be pulled down between the teeth, which is why tissue paper underneath and a straight-stitch plate help so much.

Worn dogs — teeth flattened by decades of use, or by pulling fabric through — feed unevenly. They are a replaceable part.

And the practical rule. Never pull the fabric to help the machine along. You are fighting the feed, you deflect the needle, and sooner or later you bend or break it. Guide the fabric; let the dogs move it.