When Master casts a spell that targets just him (range: Caster), it affects the communicants as well. Try Phoenix Pyre...
I haven't tried this out in a long time. It might have changed or worked differently. However, there were some crazy things one could do with Jotun Jarls carrying Slave Matrices...
Never tested that, though. Lots of work for few tricks isn't worth it IMO. Jotunheim doesn't exactly shine as a Communion nation...