Newbie question on communion

[PvK]

What's the deal with the fatigue cost 100- for the communion slave spell? If the point is to give your fatigue to the communion master, but you use all your fatigue doing it, isn't that rather counter-productive? Or is the idea the level-1 slaves pass out and then get fatigued up to 200? (Or is the idea to empower the slaves somehow above level 1 before they cast the communion spells?).

PvK