Upon reading the descriptions of Rego Vim vs. Muto Vim, it seems that the spell Wizard's Communion should be a Rego Vim spell. It doesn't seem to affect the spell being cast at all. Instead it seems to be providing a connection between the magi involved.
Can anyone explain to me what I am missing?
Thanks for your help,
Jonathan
There is no one "spell", but several, plus a target that is made larger.
I would suggest that it is Muto because it changes several spells to combine into one single, larger spell. To think of it as moving "mana points" (or whatever) around via a connection between magi is not the way that AM magic works at all- one is casting a target spell, and the other magi, casting WC, change that spell (or the roll behind it) into something larger.
Sounds like Muto to me.
The problem that I have with this explanation (I did consider it) is that the spell being cast is not made larger or more powerful in any way. The spell being cast has the same level that it ever had and the same effect.
The only thing that changes is the difficulty that the caster has in successfully casting it. The magi involved in the communion seem to be pooling their magical strength to make it easier to cast a spell. The idea that the magi are in some way mingling their Gifts is borne out by the fact that a botch on the casting gives all participants Warping points.
That magical pooling doesn't seem to fit perfectly into either Muto or Rego as described, but seems to be much closer to Rego.
I'm not arguing for a change in the spell as written. I'm just looking to understand the reasoning behind the classification.
Jonathan
Muto Vim Guidelines:
"These spells let you change other spells...
Rego Vim Guidelines:
"Rego Vim spells repel demons and other mystical creatures... also... (grants) Magical Resistance..."
Vim spells in general often refuse to be easily categorized- I'd thus suggest that it was seen as a "best fit", rather than a perfect one.
Being able to cast, say, a Level 40 Spell as if it were only Level 10 does not seem either an "only" sort of thing, nor does it seem outside the basic elements of a spell. That spell has been changed in construction, if not in final effect.
I certainly agree. And the difficulties are most evident when it comes to Muto and Rego. In generel those two Arts are soo darn close that when it comes to the more ellusive Forms they are tricky. But at least their divide is less confusing then prior to 5th 
Yes, it does change the spell (in that the TN for casting the spell is lower), but not any more than changing a spell from Concentration duration to Diameter duration ("Maintaining the Demanding Spell") or allowing one spell to trigger off of the effects of another spell ("Watching Ward"). My argument was that the pooling of magics (as evidenced by the shared warping inherent in the effect) argued for Rego rather than Muto.
Yes, I know that the arts of Rego and Muto are often hard to seperate when dealing with some forms. That's why I started this thread.
My personal thought on the subject is that it is a game balance issue rather than a purely technique issue. As a Muto spell, it is severely restricted and much less open to abuse. (In fact, the Muto restrictions are pretty much required to keep it from getting out of hand.)
I guess what I had hoped for was a bit of a philosophical discussion rather than a "you're wrong" or "that's just the way it is" kind of answer...
Jonathan
You can think about it like you are manipulating the flow of magic in the area. That can be either rego or muto. As you said, they are often tricky to tell appart one from the other.
Now, Rego, would imply that you are diverting the magical flow around and manipulating it to achieve a better result. Clearly in the area of Rego.
Now, what happens if the magic in the area is not enough in its current state to boost the spell? In that case you need to transform magic itself to boost a particular action (the other spell being cast). You are manipulating the usual flow of magic between the magi (they usually cannot "open up" their Gifts to combine their power) and boosting a second spell via a boost of magical energy coming from the Gift of the magi themselves.
It is nonsense rules wise, but it sounds cool to me 
I always thought abouit it being a resonance type of action, where the magi make their Gifts resound with each other, and by art of sympathetic connections the other spell gets a shake up and performs better (or the Gift of the caster acts better due to the resonance, whatever you prefer).
Also, remember that Wizard's Communion is mentioned to be pre-hermetic: it is Cult of Mercury spell that got adapted to hermetic standards. You can find all kind of kinky stuff and side effects when non hermetic spells are adapted to hermetic parameters.
Hope that helps. I and my players have interest in vim only to cast the Aegis and smithe magical creatures, so we have never looked at the vim spells in depth.
Xavi
I think that is a very crucial point - and to me it is easier to explain it that way than to try to distinguish between what constitutes a natural (Rego) or unnatural (Muto) change of the flow of magic.
I spent far, far too much effort trying to come up with a coherent personal rationale (and ultimately failing) for how AM magic works to believe it's possible to give you a satisfactory response.
Now, when asked that question, beyond something like I gave above, I just answer...
"How does it work? It works fine, just fine, thanks for asking..."
Sorry, but for me what you're asking is somewhere between a fool's errand and a deathmarch. Any attempt will fall short, or be so full of holes in the eyes of others...
Magic is full of mystery n' crap. 'Nuff said.