Hi Anthony,
The system layouts use a stored procedure to allow this. The system saves the document as a draft when you choose to preview & the stored procedure works out that it needs to look at the draft table.
There is some documentation on the portal that shows how to create crystal layouts, but I have found it easier to take the system ones and modify them, that way you don't miss any functionality.
If you get the report designer to look at the database expert in the system layouts, they may be able to work out what is needed.