"It only ends once, everything else before that is just progress"
I think that we know, that it's the consciousness that gets sent back in time. Just ask Des. That's why, when he "time travels", he experiences different times, meaning different dates in time. His consciousness travels, not his body.
There has been a loop, going on for who knows how long (maybe Jacob?) This loop began with the flashsideways that we are watching. Whatever Des is about to do, is going to send peoples consciousness back in time. (Instead of your body going back in time and being able to see your younger self...your consciousness travels back into your younger self) However, they are unaware of this. Their consciousness is sent back to a time where if they start making different choices, it will actually make a difference. However, instead of making different choices, the same ones are made...making this "timeloop" repeat over and over and over.
Hopefully you are still with me. Jacob, after being "badgered" about nothing changing by the MIB (seen in the incident)...decides to literally change things. He then enlists richard as his right hand man...telling richard that, even though he (Jacob) cannot not make any changes directly, he can have an "advisor" "change" some things for him, pushing events into his favor (seen in ab aterno). After being unsuccessful for so long, "timeloop" after "timeloop"...Jacob decides that his influence needs to reach out further...to the outside world...outside of the "island". He "uses" Elloise as this outside world "advisor". Now Elloise is able to send the "keystone" to Jacobs whole plan in the right direction...this keystone being Desmond.
However, this "timeloop" keeps happening...so Jacob has to reach out further and further, greatly...yet undirectly, extending his influence to change things on our losties.