Well… I found a chapter in street magic which clearly says that a spirit can materialize, use one of his physical powers on a target (in the example, it was "Guard"), but didn't have to stay materialized to sustain its power… so, it made me ask myself about a looooot of questions on powers sustaining (example : if a spirit can sustain a power from the astral space, can it sustain a power from its metaplane, while placed "standby" by its summoner, since metaplane are part of astral space ?), spells sustaining (can a magician do things like sustaining a spell cast while projecting when back to the physical plane ?), applications to the physical and astral body of the magician, with example if it's the caster of the spell/power which change of plane or if it's the target, and so, and so…
It does make a lot of questions, we found no clear answer in the rule, I think it would be better if you just take a look on the linked subject…