I'm not exactly sure. The soap is just a step in the process that I learned from people here.
The gum trag seems to seal up and almost plastise the edge. I paint a very light coat of it onto the edge. I have some small paint brushes and use my desk as a guide like this:
A lot of people have had bad results from using it and stopped. Someone here posted once about letting it have time to penetrate before burnishing would produce great results.
I have had pretty good results with it every since reading that. Slowing down our steps in this craft creates a better product, period. We all want to rush to get it finished. But after we make ourselves slow down, we (me anyway) find an intimate groove with each piece of leather. I look it over very closely. Basically, putting some love into it. This is my hobby and putting that effort into it relaxes me. Especially, when it turns out nice. I still consider myself a novice, but the more I play, the more I learn.
Always have some scrap to take through the process with your piece. This helps to teach you what to do and not do to your project.
Chrome tanned leather is very difficult to get a good edge on because of the way it is tanned. I was able to achieve this edge by taking my time and using practice pieces throughout the process.
It's not perfect, but pretty good for chrome tanned.
Hope this helps.