OK, here be da widgets.
The short one for short spools:
The tall one for taller spools:
They keep the thread from wrapping under the spool by keeping the thread heading up.
The tall one has a hollow bushing made from dowel rod inside the spool to keep it centered.
The only thread these don't work with is the C&C, Gutermann, and Sulky slick slippery embroidery thread in the those funky little tapered spools. That thread is so slick it just falls under no mater what.
Joe