ReplaceIfEditedAgainWithin), TWiki will fold together your changes. This is often the "right thing to do", as it can reduce the visual clutter of diffs.
The "Force New Revision" checkbox is a way to force it to create a separate revision each time you save.
The TWiki.TWikiPreferences variable FORCENEWREVISIONCHECKBOX controls whether this is checked by default or not.
On a related note, you can force every save to be a new revision number by setting ReplaceIfEditedAgainWithin to 0.
Related Topics: UserDocumentationCategory, AdminDocumentationCategory