Desmond is special because he can absorb huge amounts of electromagnetism, which consequently allows his consciousness to move along space and time (i.e. space-time.) In season three, Desmond's consciousness traveled between 2004 on the island and some past point, but it was uncontrolled and random.
When Charles Widmore put him in the "Box of EM," he finally sealed the bridge. In other words, Desmond is in a flux-state of EM in which he is experiencing both the Alternate Time line, and the present time line. The switches happen so often, and so instantaneously that it could be thought of as the constant throughout space-time. Unlike Daniel Faraday who cannot maintain his flux without going crazy or dying (unless he gets a hold of his constant.) Daniel Faraday is the variable throughout space-time.
Desmond is inducing near-death-experiences and deep emotional experiences to get people to remember the Island, and what happened there. Once they do, perhaps they will be able to fight Flocke remotely (in a different space-time.)