[thejeff]

I think you mean Communions, not summoning.

Communions are a way to boost a mage's power during a battle to cast more powerful spells and to spread out and reduce the fatigue.
Play around with it. Take a bunch of Astral mages, have one cast Communion Master and the rest Communion Slave, then have the Master cast big nasty spells.
With 2 slaves, the Master gets +1 to every path he has.
With 4, +2
with 8, +3, etc.

Fatigue for the Master's spells is divided up among the slaves and the master doesn't stop casting when they go unconscious, so he can quite easily kill them in a long battle, especially if there is more than one master. The more slaves you have the less likely they are to die, both because the fatigue is split more ways and because their paths are boosted as well, but only for the purpose of reducing fatigue from the master's spells.

There are all sorts of nuances, exploits and variations, but that's the basics.