My interpretation is this (let me know if this sounds about right):

A sustaining Foci is attuned to a type of spell (health, illusion, detection, etc..). If the spellcaster deliberately turns it off, they have to recast a spell (of the appropriate type) through the foci in order have it sustained again, just as if it was deactivated by dropping it or whatever. Of course, the maximum force of the spell is limited to the rating of the foci, and it has to be sustainable (i.e., you can't sustain a power bolt).

Got it in One there Rand...
Of course, you can also sustain a Permanent Spell, until it finalizes, as well...
Semper Fi...

Cool Beans, thanks TJ.

Semper Fi

Hmm, I thought it was possible to move a spell you were already sustaining into a focus. The focus would have to be active but not currently in use though... maybe I've just been reading on spirits and such too much where you can switch out who is sustaining what freely.

I was under this impression as well.

There's one throwaway line that, when describing how activating a focus is a simple action, says that it's a complex action for a sustaining focus because you need to cast the spell. It's not actually discussed anywhere else that I've seen and IMO there's certainly no harm in moving a spell you already spent a complex action on some other time into a sustaining focus. It's fairly clear that activating a sustaining focus takes no actions since just casting the spell is a full complex action on it's own.

I think that the spell has to be cast purposefully to be used by the sustaining focus - I don't think you can cast a spell, sustain it, then later decide to have the focus sustain it for you. You have to cast the spell THROUGH the focus.

SR4a, p.199

Sustaining foci are unusual in that they do not add any dice to a test. Instead, a magician may cast a spell through a sustaining focus and the focus will sustain the spell for him.

I would say that a magician can cast a spell into a sustaining focus "for free", meaning just the complex action to cast the spell, but if he is currently sustaining a spell on himself (or elsewhere) and wants to move the spell into the focus, doing so is a simple action.

It makes sense and allows some flexibility while still rewarding planning.

Yeah, don't know that it would be a free action or non-action to swap around spells, simple sounds good. But since you can move a sustained spell to a spirit and back, I don't see why you couldn't move it to a sustaining focus and back.

Thanks.

Bare in mind, I'm not sure exactly what the rules are, because they are somewhat confusing. It's a simple action to activate a focus, it's a complex action to cast a spell into a sustaining focus (so really a free action to put it in the focus), it's a simple action to move a sustained spell to or from a spirit...

I think my solution is good, flexible, and speeds up play, but I don't know that it's strictly by the book.

Well technically the simple action is to order the spirit to sustain it. When the spirit stops sustaining it after Force turns you "pick up" the spell automatically.

That said I like you simple action idea simply because no one could hope to argue that it's overpowered. Although it appears that if you can move a spell into a focus after the fact, it wouldn't take any effort at all, strictly speaking.

It becomes much more Dresden Files-esque to have people either cast the spell themselves or with the aid of their sustaining focus. I can't really remember any time where it actually worked like I had the impression either - I just thought that option was available. I guess not.