As you work on a file, Nima creates a revision history of all the changes you've made. Each of these revisions can be previewed and restored under Settings > Revision History, which is found at the bottom of the Toolbar.
Nima automatically saves your file in the background whenever it detects a change. If you want to force a manual save, you can use the following shortcut:
To restore a revision, press the Settings button and go to Revision History. Here you'll find a list of all the revisions of your current file. Select a revision to preview it, then hit "Restore this Revision." This restore point will now be saved as a new revision at the top of your list.