LostFreak reminded me of something I thought of before which I think most of us now know it but I just wanna put things straight for me at least.
When Desmond turned the failsafe key, a huge amount of the electromagnetic energy in the Swan was released into the atmosphere. This electromagnetic energy gets active every 108 minutes and pushing the button re-stabilizes it. I believe the first incident was the reason for the electromagnetism to be active every 108 which of course have a certain relation to the Island. (The 108 part). When he turned the key, Desmond was exposed to a huge amount of electromagnetism redoing Faraday's experiment on Eloise. His mind was transported into the past to see stuff he saw before. Just throwing this out, reacting with Charlie in the other time line conscious was the reason he was able to see Charlie future and no one else's. He became special since his mind now has the capability to travel into past or future without the need for a huge amount of electromagnetism.
Thinking about Einstein's theory of relativity, the island provides the source to this vast amount of energy needed to time travel which in his opinion is impossible.
Turning the Donkey Wheel "opens" a door for this electromagnetism to escape. My guess will be that the ancient civilization which I think will be a mix of the greatest civilization of all time (Egypt, Rome, Greece, India, Mesopotamia) created this donkey wheel to do the same job entering the numbers does, stabilizing the energy. The Incident which I hope we will learn about will be the reason for making the Donkey Wheel not enough for stabilizing.
I believe that the Incident will have something to do with putting metals into the Dharma's time travel machine which Pierre Chang warned to avoid.
Sorry if it is too long or too general...
P.S. I pledge that this is all based upon my thinking or other posts which I did credit. No spoilers associated at all.