I think there is a short thread on it hereabouts. Quick pro and con reviews. Some found it effective and others not so.
I gather you place the "install" files on the card. So you can INSTALL the applications when you need them and then delete them from memory when you don't. Then install again when you need it again later. So you do not actually run the programs from the card. I gather that depending on the app, the BB may reboot as it normally does with some programs when they are installed/removed.