Clarifications and interpretations of vague rule text from the Mage books.
Clarifications and interpretations of rules found in the core 2nd Edition rulebook.
When mages cast a spell, the Signature Nimbus is left behind for some time, even after the spell expires. The Signature Nimbus normally lasts for a week before fading. From mages with a particularly great Gnosis (6+), the imprints of the Nimbus stay for one extra week per Gnosis dot above 5.
How to cast spells from our world into others, or vice versa.
According to Dave Brookshaw (lead developer of M:tAw 2e) in this this post, the guideline is that you need to be able to enter a realm to cast into it. For the Gauntlet it's quite simple because the Gauntlet is porous and spells are simply Withstood by it. For other things it's more tricky. You'd need to go to a place where it's possible to enter the realm.
For example, if you want to summon a ghost from the Underworld, you either need an open gate to that particular part of the Underworld, or Death 5 to open a gate at the same time. So it would be a Space 2 + Death 5 + other arcana spell for casting sympathetically into the Underworld. The Death 5 requirement could be waived if you have an open Avernian Gate.
In all cases, the Sympathetic Range attainment (Space 2) is required as well in order to cast the spell. Emanation Realms
To cast sympathetically into an Emanation realm, you will need a Thurae. Since no known spell can create them, it's not possible to "sponateously" cast sympathetically into emanations. You will need to find the open Iris.
Sympathetically casting into the Astral requires you to be in a place where you could meditate to enter the Astral. You must spend the required amount of Mana to enter the Astral in order to cast the spell, as well as any other Mana costs the spell has. Depending on the "distance" at which one is casting into the Astral, the sympathy Withstand factor might be higher. Casting into the furthest reaches of the Anima Mundi from the Material might increase the Withstand by 1 or 2. Casting somewhere somewhere deep into a weakly-connected Temenos or Oneiros Realm might can also increase the Withstand.
Synesi (Astral Irises) can be used to cast sympathetically directly into the Astral Realm they're connected to, bypassing any Withstand from Astral "distance" (other sympathy Withstand still applies). Casting out of the Astral into the Material is also possible, although the "distance" factors apply in reverse. Cunning mages can hide in the astral and wage a shadow war against their enemies.
Paradox risk is always the worse of the realm the mage is in or casting into. For example, if in the mage is in the Underworld, Death spells benefit from -2 Paradox risk. But if they cast a Death spell from the Underworld into the Material, that -2 bonus no longer applies. See this post from Dave Brookshaw for the ruling.
Some clarifications and changes to the Paradox rules to make them a bit more exciting (not dangerous!). The rules primarily concern the handling of Paradox Conditions when containing a Paradox fails.
Paradox Conditions:
The Abyssal Taint Condition is a cleaner version of the rules for lapsed Paradox Condiions.
Some clarifications and interpretations of the way Time works, beyond what's presented in the book.
When a mage goes back in time and changes the past, the changes to history will usually be handled as a quick narrative summary, and then the present is altered accordingly. Characters' memories and actions will of course change. Time Shielding spells protect subjects against temporal alteration (and thus preserve memories), and a Mind Shielding spell combined with Time 2 can protect against memory alteration. A regular Mind Shielding spell is not enough to protect against memory alteration from Time effects. Temporal Sympathy in Detail
The key detail of Temporal Sympathy is that whatever spell you cast stays in the present. The Attainment just allows the spell's effects to reach into the past. This requires the need to have a logical combination of Combined spells when using the Attainment. Some examples:
In most cases, the most powerful applications of time magic will come from the mage traveling back in time, rather than coming up with extremely creative uses of Combined Temporal Sympathy spells. A mage that breaks their arm and wants to "heal" that can use Shifting Sands (Time 3) to rewind time and prevent the situation that led to a leg being broken.
People or objects in the past that start altering history pick up a temporal aura, visible under Time mage sight. The more drastic the alterations, the powerful the aura. The core book references this specifically for mages that travel back in time, but the same principles also apply to Combined spells cast using Temporal Sympathy. The guidelines below reference the past as if it were the present, for ease of comprehension.
With the use of time magic, it's very possible to cause what Sleepers would call a temporal paradox. If a mage prevents their own existence, they were never alive to cast the time travel spell in the first place, so how could they have gone back in the past? Like all the Fallen World, causality is part of the Lie. Mages are insulated against what Sleepers would call temporal paradoxes. A mage who prevents his own birth will return to a world that doesn't know him (core book p. 186). This principle forms the basis for handling of more complicated paradoxes.
Example: A mage travels into the past with Shifting Sands to protect their cabalmate from death at the hands of vicious attackers with shotguns. She travels back and casts a spell to make her cabalmate's skin bulletproof. The assailants now find themselves unable to injure their original target. They decide to change targets, and wind up killing the mage that would later travel back to cast the protection spell. This means that the mage was never around to travel back in time ot cast a protection spell on their cabalmate. The solution to this temporal contradiction is:
Causality doesn't matter; it's Magic™.
Mages might attract unwanted attention from the past, present, and future if they start meddling with time too much. This can lead to situations where mages are continually entering the past to attack each other, undo each other's time spells, and such. These situations are best viewed as a scene unto themselves, as sort of a "meta timeline." Rather than getting caught up in attempting to understand such temporal loops, these situations are usually handled with an custom application of the down and dirty combat rules: that is, a contested "spellcasting roll" using the Time arcanum will determine who is the ultimate victor in a temporal struggle.