You are using the tool in a wrong way.
1. If you have choosen the foreground select tool, the cursor acts like the selection tool. Select the object that you want to cut out roughly.
2. After selecting the object, the outer mask is shown. It seperates the object from what is "definitly background".
3. The cursor changes to a round paintbrush.
4. Paint on the object, that should be cut out. Paint in just one stroke, because after stop painting, the time consuming cutout alghorithm will run, and if you are not ready with painting, you have to repeat it. The painting says the alghorithm, what is "definitely foreground" (the object).
The alghorithm (siox or matting) is for calculating in the space between "definitely background" and "definitely foreground" which pixel belong to background and belong to foreground. On that calculated border, the cutout of the result will be set.
5. Press Enter, and the result will be put on a new layer, making the background transparent.
The matting (RGGJAN) alghrithm makes besser results than the siox alghorithm, but it changes some colors around the calculated border to enhance the result.

Well, I actually did do that. After Clicking Enter. The background did not disappear. It just left my selection with the ants around it. So, i just copied and pasted, what was selected.. onto a white background. Erased the edges. Then, I made the white background transparent, with my selection.

I do appreciate you taking the time out to explain this tool to me. Thank You