Now this is an idea that I've been toying with a bit and I think I have most of my thoughts on this subject together now so I'm going to give a shot at explaining my theory.
Now since I was having this debate already I'll just put it in here, I haven't made any assumptions that Jacob is not a man. But I have made the assumption that Jacob is more then a regular person. Like Miles and Hurley and Desmond are people but more then regular people, Jacob seems to have "powers" so to speak that are better then any of those guys.
One of Jacob's powers seems to be immortality or perhaps a better way of putting it would be agelessness (i.e Richard, and if he can cause Richard not to age why not himself). So if Jacob can live for hundreds of year we can all agree he's got a lot of time to try things.
The next part of this theory relys on the assumption that Jacob most likely has knowledge of the FDW and how to get to and from the Island (seeing as he has done it to go get Ilana and has brought others there like the ship in the Incident). Now we all know the exit for the FDW is in Tunisia, but since Jacob seems to know how to get to the Island and can seemingly do it whenever he want he pretty much has free reign to turn the wheel as much as he wants.
Now, the last part of this theory relys on another assumption, the assumption that Jacob can control, at least to some degree, what time periode he comes out in when he turns the FDW. So, if Jacob realised that the events in this loop didn't play out exactly the way he wanted he just goes and turns the FDW, then he just make what ever minor adjustments he wants then sees how events play out this time.
This theory also goes back to the incident when Jacob uses the phrase "It takes a long time when you make the threads" I know he was refering to his tapestry but honestly I took this to mean that Jacob is the one pulling all the strings, strings he made no less, of the Losties and making changes in their lives that lead them to be the people he wants them to be.
If you look at the incident I belive that the changes we were shown were changes that Jacob had to make in order to get the people where he wanted them, for example looking at the incident, I belive that Jacob has gone through quite a few loops to know exactly what events where and when that he needed to influence in order to get his Backgammon peices into places (that's right I said it!).
Now that we've gone through my long boring theory made possible thru time travel and agelessness, let's open this up to debat only to be riped appart (I love to see the holes in my theories so have fun destroying this guys).