How is the bomb Jack dropped able to bring them back in time?

ah...that's a very long story,Ready?Fasten ur seatbelt,here we go:
how did oceanic flight 815 crash in the first place?Remember?it was because of the electromagnetic energy in the angsa, swan site that was needed to be discharged every 108 menit so it doesn't build up too big,but on September 22nd Desmond follows Kelvin out of the Hatch and accidentally smashes his head against the Rocks,killing him,which causes him to be too late to push the button and 815 crashes.
but why do they have to push the button?
According to Dr.Marvin Candle it is because of an Incident that happened before in the hatch.
According to Daniel it's a leak of energy from a big underground pocket of energy and that if it's destroyed before that leak happens,then there won't be any 108 menit Discharge and there won't be any Kelvin head smashing and finally,no Flight 815 crashing.