If it isn't a glue, then it must be starch.
Or <g> it is a pretend solution ... since, if you took water, put it on the line and folded as they instruct, between the memory in the cotton fabric, and the fold you are making while damp (where do wrinkles come from?)
the results would be the same if you used a damp finger.
I would think you could accomplish the same thing with a Q-tip and liquid starch or plain water.
If I understand the concept correctly, the starch would make the edging for applique very crisp.