This theory explains the purpose of the Swan station, the dharma supply drops, the radio tower's purpose, Black Rock in the middle of the island, Desmond sailing in circuits, the plane crash, the ballon crash, why Michael and Walt were able to leave the island, researches on Valenzetti equation, Faraday's watches.
The Swan pushed the island every 108 minutes like a pendulum, the island travelled into the past came back to the present ( 2004 ) and swinged further into the future. Forth and back in time like a pendulum.
That's the reason why they built the radio tower. It sent a permanent broadcast into the world and when the island swings into the year 1984 for example, dharma outside knew were the island was and when it will be back again ( radio tower=like a lighthouse ).
They calculated island's position on the timeline based on a constant pendulum swing and the radio transmission they received when the island reached off-island dharma time. Making these calculations their plane was always able to drop their load just in time so it reached the island when it swung back the other direction.
I guess the island made a full pendular movement in 216 minutes, 108 minutes into the past and after that 108 minutes into the future ( island time 108 minutes, outside maybe 108 years pass or even more ).
Every 108 minutes the island reached its starting point (our present time) and if nobody pushes the button, the island stands still in the present and the enemys of dharma can reach the island. That's why they decided to destroy the island if nobody pushes the button anymore (dead-man's button), so the enemys can't occupy the island (which would lead into the destruction of the world because the enemys -boaties- have evil plans with the island.)
Desmond turned the failsafe key after 108 minutes ( or rather one minute too late so the island stopped a few minutes in the past ) and the island wasn't destroyed. But the island is now a little bit asynchronous in time ( -->Faraday watches ).
Anyway that's why Desmond travelled in circles with his boat, because the island was still swinging when he sailed away, shortly after the Swan's implosion the swinging ended and Walter and Michael were able to leave the island heading direction 325.
The Black Rock came to the island while the island was swinging through the past, maybe it was just on the "wrong" or exact position when the island came by and pulled it onto the island. Or dharma made a mistake while playing around with the Swan computers and which lead to an emergency braking in the 1880s dragging in the ship.
Would also explain Henry Gale's ballon and also of course flight 815. Every time when the pendulum is stuck it pulls something onto the island from that timeframe.
This swinging would possibly explain why dharma tried to change the Valenzetti equation result 108. They wanted to expand the 108 minutes, so they can reach further into the future and/or further into the past. They want to reach a specific date in time where they can change something important for humanity. Changing the Valenzetti equation would result in a wider range in time.