According to the manual for the LS30, it has a darning plate to cover the feed dogs when FMQ'ing or darning.
http://www.brother-usa.com/ModelDocu...30_EN_1097.PDF
You will use your walking foot with feed dogs uncovered (normal) when stitching straight lines--in the ditch, diagonals, diamond/crosshatching, or sewing on binding. You''ll put the darning plate & darning foot on when doing swirls or loops--like drawing on your quilt using the sewing machine as a pencil.