The purpose of a focus on form is to highlight new forms for learners and make them aware of the long term need to master these forms. So it's not a matter of a blitz on, for example, the passive, in the hope that learners will assimilate this new form as a result. It's more a matter of making them aware of the passive and giving them some insights into its use. As a result of this they will be more aware of the passive in their future reading and listening and so kmore likely to begin to use it.