Full backs normally hang back for the most part in the back of the field. For defense, a full back is your last line of defense, and it's their job to make the tackle if no one else can. On offense, a full back is a great player to insert into the back line to overload the other team's defense. They insert into the back line to help run the ball. The also can support the weak wing on offense. If for any reason a back (usually flyhalf) gets drawn to the weak side, it's the fullback's job to cover that hole.
