During Faraday's conversation with Chang, he mentioned that the energy at the Orchid station was extremely powerful (enough to kill somebody who had gotten too close).
He then said that the energy at the Swan station site is nearly 30,000 times as big. That is a colossal amount of energy. If the energy at the Orchid is strong enough to move an ENTIRE Island and manipulate time and space, then the energy at the Swan is powerful enough to completely annihilate the entire world.
We have already known that for a while though -- Kelvin mentioned that he was "saving the world" by discharging the electromagnetic analomy from becoming too large. Chang also alluded to this in the Swan orientation.
Faraday also mentioned that the Incident occurred when the DHARMA members drilled into this energy pocket, and it had to be covered with concrete, like Chernobyl.
So now we know the Island has two main pockets of energy: at the Orchid site and the Swan. Both of these energies have something in common: the ability to manipulate time. But they manipulate time in two different ways:
- The Orchid station (when the frozen wheel is turned) manipulates time in a physical aspect (i.e. the time shifts, etc). When this occurs, everyone moves in time (the Others are an exception, but that's a different topic). Also, as seen in the Orchid video outtakes, a single bunny could be moved in time (physically, not by consciousness).
- The Swan station manipulates time in a different way. When Desmond was exposed to this energy, he shifted to 1995. After he lost consciousness, he came back to the present (2004) with a new ability -- parapsycology (ability to see the future) and later time traveled again in consciousness (in The Constant). This most likely did not come from a direct result of turning the key, since we know that Minkowski had similar experiences. Desmond's unique abilty that he inherited from the Swan energy was being able to 'change the past' and also predict future events.
My theory is that this energy at the Swan did something to Desmond -- it made him special. It gave him the miraculously special ability to actually change destiny; to change the past. Instead of the standard rule of WHH applying to him, Desmond actually acquires new memories (i.e. Faraday at the Swan) and can change events (i.e. meeting Daniel at Oxford)
Faraday seemed pretty convinced by his research about the WHH theory: "You cannot change anything; you can't. Even if you tried to, it wouldn't work... whatever happened, happened". But then he had a change in mind -- the variables could change the past. I think there is only one variable, and that is Desmond Hume. So why is he a variable and Jack/Kate/Sawyer/Daniel etc. not? Because he's special (turning the key/vast electromagnetic energy exposure/finding a constant/changing the past/etc.)
So when Desmond turned the fail-safe key, he was exposed to the immense electromagnetic energy pocket (when the discharge occured and the sky turned purple) which gave him the ability to alter not only his destiny, but the destiny of others as well. I remember thinking that Desmond was definitely dead when he turned the key in Live Together, Die Alone -- but instead, he inherited the unique properties of the energy and became almost like a "superhero" (as Hurley would call him). This is why Desmond is so special and can actually change the past.