How do I create a "knockdown" stitch in Brother PE-Design 11 embroidery digitizing software? Is it called something other than "knockdown"?

 

Do you mean the bottom fixing stitches layer?

The stitching that holds down the nap on a "fluffy" fabric.

Use the shapes option / tool, draw a rectangle (or whatever), then right click in the shape and change it from fill stitch or whatever to Net fill stitch.  Then in the region sew area of the window, change the spacing to what you want it to be, 2mm, or whatever.  You can also change the shape of the knockdown stitches / area.