From UO Evolution
While the name, Resisting Spells, suggests that it helps with resisting all spells, this is not the case. This skill helps you lessen the severity of spells that lower your stats or ones that last for a specific duration of time. It does not prevent direct-damage spells, like Energy Bolt or Flamestrike.
At higher levels, the Resisting Spells skill also benefits you by adding a bonus to your minimum elemental resists. This bonus is only applied after all other resist modifications from what you're wearing have been calculated. It's also not cumulative. It compares the number of your minimum resists to the calculated value of your modifications and uses the higher of the two values.
For instance, if you are a Grandmaster, your minimum resists will never go below 40 (see table below). So if you happen to be wearing armor that adds 20 to each resist, the two values will not be added together; instead, it will use the highest value, which happens to be the number that your Grandmaster status gives you: 40.
If someone who doesn't have any points in Resisting Spells wears that same armor, the value of their resists will be at 20, since they will not get the resistance bonus from having the skill. The calculation of the bonus to resist is done with the following formula:
40-109 resist spells
110-120 resist spells
Publish 48 of November 27, 2007, increased the effectiveness of Resisting Spells against the Necromancy spell Blood Oath. Resisting Spells now reduces a percentage of Blood Oath damage by the following formula:
(Resisting Spells Skill / 2) + 10 = percentage of damage resisted