The Quantum Field Focus Controller is a piece of technology aboard some Odyssey-class and Vesta-class starships that allow the ship to keep it's shields and communication systems online and at 100% during Slipstream travel. Without the QFFC, ships traveling in a slipstream corridor would find communication almost impossible and shield strength severely diminished.
It can also be used as a beam intensifier. If the ship is heavily damaged and the Field Focuser is still online, power from the phasers and shields can be rerouted to the main deflection system and be used to either create a shield bubble or a high intensity phaser beam which could be sustained for as long as power can be routed to the deflector array.
*Theatre Note* Irregularities within the LMC make this device respond unpredictably. Sometimes it functions other times it does not.