What is the difference between zero leading and zero lagging?
Zero power factor means that the load connected to the alternator is either purely capacitive (zero leading) or purely reactive (zero lagging), therefore, there's no KWatt (real power) consumed by the load.

As you know, the armature mmf (Fa) depends on the armature current (Ia). when the load is purely resistive, (Fa) is 90 electrical degrees behind the field mmf (Ff).

When the load is purely capacitive (zero leading pf), (Ia) advances by 90 degrees, thus (Fa) is in phase with the field mmf (Ff) and in this case it helps in increasing the resultant mmf (Fr). That's why we say it is magnetizing.