Full defense is a trapping of Rapport seemingly only in social rolls (see "closing down" trapping which is obvious when you're doing full defense. Other types of defense such as deceit or discipline applied to social attacks don't seem to have a full defense mode, they're already "active" I guess....)
For other kinds of defense, it is tied to making a "defense roll" against an attack or maneuver, which normally means Athletics (dodging), Discipline (mental defense), Fists (close combat defense), Weapons (melee defense).
The important thing (regardless of how social skill work) is even if some other skill is being attacked (like maybe a gravity based attack on might) you get a defense roll. In a block, there is no defense roll, there is only the roll to establish the block strength. So full defense is only useful in conjunction with a block if by doing so your defense roll is higher than the block strength, and the block strength is beaten by an incoming attack.