This is not just for WYSIWYG, but generally for the edit pane.
AJAX provides the facility to background save - this would be a boon for twiki as it would save the possibility of losing work if the browser crashes or is inadvertently closed.
Confluence has this feature.
This is now available in the latest TinyMCE-3.5.8 integration.
-- Peter Thoeny - 2013-10-02