The spoon busk was a specialised kind of busk—the rigid element of a corset placed at the centre front.
As its name implies, it was shaped like a spoon, with the bottom part of the busk widening and taking a dished form.
The spoon busk allowed a greater reduction in waist size without producing a bulge of flesh at the bottom edge of the corset.
This was a problem experienced when corsets with straight busks of even width were tightly laced: as the flesh of the abdomen was, essentially, squeezed out of place and appeared where there was no pressure.
The wide, dished part of a spoon busk accommodated the abdomen, and at the same time compressed and controlled it.