$ FADH_2 $ can be compared to a basket with two hydrogen ions and two electrons. It can transmit these electrons so act as reducer . The $ FAD $ is comparable to the empty basket. It can recapture both electrons and hydrogen ions (dehydrogenate!), So act in oxidant
1 : adenine 2 : ribose 3 : flavin 4 : diphosphate group Hence the name: FlavinAdenineDinucleotide
The 2 $H^+$ and 2 $e^- $ exchange is taken place in the isoalloxazine group of flavin .