The walking foot uses a combination of the feed dogs up and engaged, moving the bottom layer along, as the walking foot is "stepping up and over" on top to allow the top layer and batting to move along with the bottom layer; then the foot comes back down and the machine makes a stitch. This allows you to make essentially "normal" lines of stitching through the thick layers with minimal shifting.
The feed dogs lowered position is used with FMQ or free-motion quilting so that you can move the quilt around freely as one layer as you stitch.
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