The use and loss of personal power.

[HerenIstarion]

, both good and bad

well, not both. Here Tolkien (being devout christian) involves religious princeple - to keep something, one has to give it away with free will. So, good maiar, giving away their power, keep and even increase it, and corrupted ones lose it, for their main reason of giving it away in the first place is gaining more power. the motive of Sauron's in creating the ring was to become the Lord of all the created world, and so Melkor's, they put in to gain, while Yavanna, or Aule, or Manwe create out of love and as an imitation of they Creator, Eru, to whom (i e Aule) they offer their creation as a gift (though the ability to make is His gift to them in the first place, so it would be more correct to say, they return their gift), and therefore preserve the ability to make. Corrupted maiar are inward, their main attention is to themselves, and good, or normal ones outward, to the rest of the Ea and, chiefly, to Eru.