Warren
When you open an existing dwg for modification Icad (& Autocad) creates a BAK file. This is a copy of the dwg before the current modification & is stored in the same folder as the original dwg.
Try creating a new dwg with a few lines, circle ... whatever, end the session for that dwg. Then open the same dwg and delete a few bits & end that session. Check the folder & you should find both the dwg & bak files but with differing times. You can rename the back as a dwg (change the file name as well) and open the renamed dwg & you should find the dwg before the last editing session.
If you 'save' during an edditing session then the bak file will be updated for that time.
Hope that helps
SteveN