It alternates using between fuels when one fuel depending on various conditions, for example a vehicle containing 2 fuel tanks - one with diesel and the other with an alternative or a hybrid fuel like E85. You can use diesel, the most expensive fuel only to start your vehicle, then the other fuel to continue running. Thus you can save not only fuel and energy but also reduce the carbon emissions by using such hybrid fuels.
The main advantage of FFV is that they allow both the fuels to work at the same time.In other words,you can supply both the fuels to the combustion chamber at the same time.
Another advantage of these flexible fuels is, you need not use only 2 fuel types, it may contain as many as range of fuel mixtures. Usually gasoline and natural gas are used as alternatives.
You can learn more information on flexible fuels, it's economy, energy capacity, environmental and healthcare effects, etc. here. You can also find the places where exactly this flexible-fuel methodology is implemented in various countries.
