1. The first curve will make it resemble more of a spoon busk, so it wraps around and slightly underneath a full lower tummy, and helps pull it up and in.
2. The second curve will bring in the lowest tip of the busk to prevent the look of a
distracting "pelvic protrusion".
3. The third curve to the busk is creating a concave “dished” profile to make the side-view look more curvy and slender.
4. The fourth and last curve will push outwards the very top edge of the busk – this will help those who have sensitive sternums, as the top of the busk will put less pressure on your diaphragm / not poke into the solar plexus area.
Bend each side of the busk separately or together?
You have the option of bending each side separately or bending both sides of the busk together. If your waist trainer has a boned under busk that has an extra-wide, stainless steel bone under one side of the busk, and the actual busk itself is a very flexible, standard width busk, I would first manipulate the side with the under busk – then I’d put the busk together and see if curving the other side is even necessary or not, because sometimes a flexible busk will bend to the curve of the stronger under the busk.
If your hands are strong enough, I'd curve both sides of the busk together, clasped closed, so that both sides of the busk have the same amount of curve – this will ensure that the loops and pins will always line up. You'll want to support the areas where the loops and pins are riveted in, so the busk doesn't break there or the pins don't fall out. What you're aiming for is for most of the curve to occur between each bracket and not much right at the bracket.
If your hands are not strong enough, you can curve each side of the busk separately – it is the more careful way of doing things, but it also takes longer to make sure that both sides of the busk are curved the same amount, and that all the loops and pins line up exactly.
Does the type of busk matter?
If your waist trainer has carbon fiber bones adjacent to the busk (which will only be included by special order in a custom waist trainer), don’t even bother trying to bend it. It will be too stiff to manipulate significantly with your hands, and carbon fiber is designed to be strong but relatively brittle. Rather than holding a curve, poor quality carbon fiber would rather shatter – so if you want a curve to your front, you will have to remove those carbon fiber bones and replace them with steel.
A wide stainless steel busk and a spoon busk may be more tricky to bend, but it is possible. Flexible standard width busks are relatively easy to bend.