MODx Revolution lacks a preview of unsaved changes. For editors this is a rather important feature to check if their changes are correctly displayed and it's easy to do. No need to unpublish, save and view the resource. Just click 'Preview' and a MODx Window will show you all the changes.
The way it works is pretty easy: When clicking the 'Preview' button and OnBeforeDocFormSave is triggered all current (saved) data will be stored in a cache file, the resource will be saved with the new data. If OnWebPageComplete is fired, the saved data will be replaced with the previously cached data.
So for a short period the actual preview will be live!