I swear on all things unholy that the descriptions in Ars Magica 5th edition on how Magic Resistance (MR) and Penetration work are confusing. As a roleplayer’s PSA here is my breakdown of how it works, with a worked example that explores some of the edge cases you’ll encounter.
(I’ve checked with the gurus in the Ars Magica official forums, so the worked example below should be accurate.)
So how does MR and Penetration work in Ars Magica?
The simple explanation is that the penetration total for a spell must be higher than the target’s magic resistance. If they have no MR, then the spell effects are resolved as normal. So the Penetration total must exceed the MR.
As a basic principal the more raw power the Magus has to cast the spell the better, however that power is used for the generation of the spell effect and also for determining if it can pierce the Magic Resistance of the target. Essentially the more power the Magus uses in generating the spell, the less power they have to defeat any MR they might encounter.
That might seem odd, but it makes sense when you consider that most mundane targets (most normal humans) have no magic resistance at all. So against those targets the caster can opt to use their best very powerful spell (such as an instant death spell) instead of a far weaker spell that might just hurt or paralyse the target. The lesser powered spell might harm a creature with MR, as more of the remaining potential energy is being used to penetrate the creature’s resistance.
However almost all of the supernatural creatures, Magi, and some holy men do have magic resistance from various sources, and their MR must be overcome before considering the effect the spell might have. Thus the Ars Magica rules were updated in 5th edition to demonstrate that MR is valuable and something that Magi must consider when fighting with magical enemies.
So the penetration is the remainder when spell’s power level is subtracted from the power the magus generates. This means that sometimes the penetration value (the remainder) is less the magic resistance of the target, and therefore the spell is repelled. Sometimes the remainder is negative, which means that while the spell was cast successfully, the spell could only affect creates that have no magic resistance at all. i.e. It would only affect mundanes.