"You said it was a steel boned corset, but no steel bone could be this flexible!"

Yes, yes it can.

Flat steel bones (generally used for the front busk closure, as well as for the very last bones at the back panels of the corset) are rigid indeed. But spiral steel bones are flexible. And, for good reason! The flexibility provided by spiral steel bones (throughout the paneling of the corset) allows the corset to mold to your natural curves and further/better accentuate them; something that cannot be achieved so easily with a corset made up of entirely flat steel bones.

Best way to tell if you've got a steel boned corset? The magnet test:

When reading instructions on how to put your corset on correctly, many instructions go something like this:

"Untie the knot at center back and loosen the corset to its fullest capacity, without undoing the knot"

..? But, uh, I'm sorry, which knot am I not supposed to undo?