The first questions is what are stays and the simplest way to explain them is “stay here”! Stays use the concept of “path of least resistance” this is demonstrated in the concept of windows., or cut outs. Stays can be made from any material as long as they can control the material. Stays allow functionality of a bag and exaggerate design concepts, such as the fan or pleated gusset in handbags, same thing used in the same way on different objects. To create a fold you need stii and soft, the soft is where the bend will take place, the path of least resistance.
Here are examples of stays.