Git workflow: commit != OS save

I am unclear on how git is supposed to work. I have looked at the official docs, and many tutorials, but I have not seen an answer to this question.

When you have a new feature or bug fix you want to work on, and you create a branch for that purpose, what are you supposed to do with the interim files while your work is in progress? Git is only saving 'snapshots' of various versions of your working files. But in order to see if any of your edits work or are worth keeping, you have to test them. And you can't test them without them being saved to the disk by the operating system. Once you have saved these files, they are not 'snapshots', so how does git save or restore anything to the original condition of master once the files on disk have been 'permanently' saved in order to be run and tested?

There must be an answer, but I haven't found it. The only tutorial I have found that even references saving by the OS was http://ift.tt/1sIKL9L, and that only in reference to a merge conflict.