In fact, as happens so frequently, a mistake (costing me hours and ripped-back yarn), recently explicated how they work to shape a collar.
OK, they're not actually adding depth to the collar, as I see it, though I suppose you could view it that way. They're acting like notches in a sewn curve (i.e. the notches you clip into a bust curve of a princess seam to get it to lie properly). They're giving shape to the collar by adding little pie-shaped wedges around the outer curve.
When you knit you can't clip your yarn to achieve the proper turn of fabric and drape, so you have to introduce appropriate "shaped-space" into the construction of your fabric.
Here's the very general, unadorned anatomy of the shawl collar short row:
- Row 1: You knit over to a specified stitch, wrap around the next stitch, turn your work around.
- Row 2: Now you knit back to where you started at the beginning of row 1. You have a weird little row that doesn't actually affix to anything, except by a wrapped loop. This is the short row.
- Row 3: You knit back to that wrapped stitch, pick it up and keep going till you get to the end of the "real" row. At this point you've introduced an extra wedge of a row that doesn't actually add to your row count, though it does shape your garment.
If you want a long rectangle of (ribbed) fabric to mold into a shawl-curve, you'll need to introduce wedges of different widths (i.e. "notches") at various intervals in its construction.
Does this make any sense to y'all, or is this the kind of thing that needs to be seen to be believed?