On FAST ESP, there used to be a technique to get back all documents in your index – enter the minus character, followed by a term that wouldn’t be in your index.

For example: -394kidfdkadkfl2k2

This would bring back all documents.

In FAST Search for SharePoint 2010, this does not work.

The trick instead, is to simply enter the hash character ( # ) by itself / without the parenthesis in the search query textbox.