This module is very similar to Revision Moderation, and uses much of the code therein. The major difference is that instead of a moderater releasing the 'future' revision, any user with appropriate permissions can do this.
This effectively allows the user to save their work as a 'draft' until they are sure that they want to commit/publish the changes and make them live. The term 'draft' is merely a unique revision, a future revision of a node.
You can only enable Save as Draft once a node has been saved, therefore the Save as Draft checkbox is disabled on node creation.
For editors to see the Save as Draft checkbox you need to either grant "administer nodes" permission, or you can use Override Node Options and grant "override [nodetype] published option" for the node types that you want the editor to save drafts for.
Please leave feedback, requests and patches in the Issue queue.