I asked this on DumpShock and got some good discussion, but nothing harder than "my interpretation / opinion", so I'm going to see if I can find something more official here:
What circumstances prevent a mage from continuing to sustain a spell?
From page 184 (20A edition core)
Many spells can be sustained for as long as the magician is willing to concentrate on the spell, as noted in the spell’s Duration.
also from p184
If the gamemaster chooses, certain circumstances may threaten to break a magician’s concentration while she is sustaining a spell, such as
taking damage, full defense, dropping prone, and so on.
I take this to mean that in situations where the magician is unable to concentrate (going to sleep for the night, for instance, or otherwise going unconcious), the sustained spell dies.
What about sustaining spells while being possessed? If my mage casts increased reflexes on the Street Sam, and then fails his int+will roll to fight off an enemy task spirit, does the Street Sam stay boosted, or am I unable to sustain it?
From SM p102
The mind of the vessel remains in whatever state it was when possession began; if conscious, it becomes an impotent witness locked inside its own body for the duration.
Does this "state of mind" of the vessel magician include concentrating on sustaining the spell? If so, can I cast increase reflexes on myself, have a spirit possess me, and have the spirit romp around with a boosted 4IP? Or does my physical mask spell drop upon possession?
Since (from SM p101)
the vessel and the critter are considered a single dual-natured entity for the duration.
Does the spirit take a -2 penalty from sustaining the spell?
Under counterspelling sustained effects (20A p185)
Counterspelling also allows a magician to dispel a sustained or quickened spell, canceling its effect. The character must be on the same plane, must be able to perceive the spell she is targeting, and must use a Complex Action.
Does this imply that sustaining too needs to be done from the same plane as the spell effect (so Astral perception / projection might disrupt physical spells), or simply contrast to countering spells as they are cast, where the magician can counter spells even if (s)he cannot perceive them.
Under channeling (SM p54)
Through channeling, a magician who was willingly possessed can find a balance between the two minds (the spirit’s and her own) occupying her body and achieve greater control. The Channeling magician can use her own skills and has fine motor control over her body while enjoying the enhancing benefits of the Possession power (see p. 101). Control is still shared, however, and the magician is unable to tap the possessing spirit’s powers without expending a service.
So it seems like under normal possession rules, we have this two headed monster, where the spirit head controls the boosted physical attributes, the mental attributes, and the special attributes, while the other (vessel head) has his mind, the "impotent witness", which cannot access or control any of the above-mentioned. Channeling seems like it brings together these two heads, so that the channeling vessel head can offer it's skills and access it's own Magic/Edge.
I'm still wondering why the un-channeled vessel head couldn't continue to concentrate on the sustained spell "in the state it was in when the possession began", with perhaps a concentration check.
What about with a focus? (20A p199)
...a focus continues to operate as long as it
is on the owner’s person, be it worn, carried, hand-held, or in a pocket or pouch
I saw no clarifications from the SR4 FAQ
Thanks in advance
--gargaM0NK