This skill doubles the speed at which your affinity changes, but causes you to gain XP from using spells much slower.
One example is the Fling spell. This spell, without the skill, will increase your air affinity by 0.05%. With the skill, however, you gain 0.1% air affinity. As without the skill, affinity will still decrease if you use a spell of the opposite element. This decrease is also doubled.
Only get this if you know what you are doing, or if you are a high level. This effect is permanent if you go above 50% of any affinity.