Martin Q. Blank said:
At the end, everyone rides the kick and wakes up from the dream except Cobb and Saito. They're both stuck in limbo. Cobb finds Saito in limbo and they both remember to come back.
How were they still connected? To even communicate with each other in limbo. They wake up together on the plane and the machine is gone.
I've thought way to much about this movie over the last decade and a half.
(1) The machine is what creates the shared dream. The mechanics of how the machine works aren't explained, other than that it requires sedation.
(2) They have enough sedation for the entire flight.
(3) If under while the machine is going on, a sensory kick can wake you up into the level above it. They have scheduled the kicks to wake them all up one level after another to bring them all the way back to reality, but also to maximize the time they have on the plane and in each level to accomplish everything they have to do.
(4) If you "die" in a dream while under heavy sedation, your brain takes you to your subconscious. But because you're on the machine, the subconscious level is shared. Ergo, Saito and Cobb are together there. You can't be woken up by a kick in this space because it is the subconscious. You just have to wait it out until either (a) you die, or (b) the sedation wears off.
(5) It's not specifically addressed, but if you don't do that and just live out the entire time in your subconscious without trying to get back to reality, it scrambles your brain when you do wake up. Like Saito would have woken up in the plane even if he didn't believe Cobb, but it's almost like he would have woken up as the old man he had just lived 200 years as.
So, Cobb, using Saito's phrase, convinces Saito to accept death to return to reality, so when he wakes up after the sedation wears off, he'll remember the deal.
