Simple explanation of synergy? How weed and beer taken together mess you up WAY more than twice as much of either of the two =)
*Probable* explanation of this particular synergy: itra and terbinafine both incapacitate fungi in different ways each, but when the effects occur at the same time, that's enough to kill them dead. Sorta like one kneecap slows someone down, but two of 'em, and they ain't gonna be moving anywhere at all...