HOW WORD CREATES THE BACKUP COPY
Checking the "Always create backup copy" box on the Save tab of the Options dialog box ensures that you will always have a backup copy of the document you're working with. This backup is a copy of the next-to-last saved version of the document, so it's not necessarily the same as the current saved version of the file.
You can't make the backup copy exactly the same as the current version by saving the file twice in succession (for example, by pressing Ctrl+S twice), because Word will save the document only when it is "dirty"that is, when it contains unsaved changes. After the first save, the document is clean until you change it. Still, by saving your documents frequently, you can keep the backup copies very close to the current versions.
When Word saves a document and fast saving is turned off, Word actually saves the current document to a temporary file in the same folder as the active document. When the save is complete, Word either deletes or renames the previously saved version of the file, freeing up the file's "real" name, and then renames the temporary file with the real name.
Word performs this apparently unnecessary shuffle to reduce the possibility of losing changes to the file while the save operation is happening. This loss is unlikely to occur but can be catastrophic if it doesthe entire document may be corrupted.
If you check the "Always create backup copy" checkbox, Word renames the previously saved version of the file as the backup copy rather than deleting it. The temporary files can provide a safety net if your document gets badly mangled.
Word's fast-saving feature appends the latest changes to the end of the file instead of writing a whole new file, so it doesn't work with the "Always create backup copy" option.