The text for arcane arrester reads as below:

When affected by a spell (including a critter’s Innate Spells), the character—and she alone—treats Force-based effects (damage,
paralysis, etc.) at half (round down) actual strength. Note that the actual Force of the spell is not actually reduced. For instance, a
character with Arcane Arrester targeted by a Force 5 spell would resist it as if it were a Force 2 spell, though the spellcaster could
still add hits to improve the effect. Arcane Arrester cannot be combined with Magic Resistance (p. 79, SR4). This quality can
be taken by characters with a Magic attribute.

So does this effect healing or other spells/critter poweres that do not have a force? Also, what happens if the character with arcane arrester fails to resist the spell at the reduced Force? Is he subject to the full force of the spell as cast or is it still rounded down?

Since it explicitly states Innate Spells and not critter POWERS, I would suggest that the powers themselves (Compulsion, Movement, etc) are not affected by Arcane Arrester. In essence, only Sorcery-based effects are reduced. If the target fails to resist, it should still be considered at reduced force. For example:

Dudely has Arcane Arrester and Willpower 4.
Snidely hits him with a Force 6 mana bolt, and gets 3 successes.
Dudely rolls 2 hits on Willpower. Ordinarily, this would do 6+3 (-2 from willpower) Physical Damage, so 7P. Since he has Arcane Arrester, he actually suffers (6/2)+3 (-2 from willpower) Physical Damage, or 4P. From the writing, it seems only the effective force is reduced, so it'd be possible to Roll 4 hits on a force 4 mana bolt, and still do 2+4 hits damage, rather than 2+2 hits as would be the case if the force was actually 2.

Nice one ^^

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