Saito dies in the third layer from his wound in the first layer obviously. He becomes trapped in limbo. Cobb gets stabbed by Mal in the fourth layer, and says he has to stay to find Saito (because without him he can't return to the United States.) My initial interpretation is that Cobb "dies" from his stab wound, thus going into limbo where Saito is. I interpret this to be another level from all the others, because at the end, Cobb has trouble recalling why he is going to find Saito. They talk about the agreement, and Cobb mentions a "leap of faith." The falling sensation is what wakes them up. Now if you'll remember that each successive layer gives you more time to return using the "kick" as the previous one, then Cobb and Saito theoretically have more time to return using the kick. Thus, they create the falling sensation and use the kick to return to the plane, and back to reality. The little top or whatever it is was wobbling at the end.
I'm sure there are holes in there I'm not thinking of. Plus, if you also subscribe to the notion that the whole thing is Cobb's dreams plus his sub-conscience then this is null. I think with movies like this we tend to over analyze stuff, but some of the stuff is plainly explained in the film.
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