I recently re-watched Christopher Nolan's 'Inception' and found myself puzzled over a specific plot detail involving Cobb and Mal's totem. In the movie, it's implied that Cobb did something to Mal's totem, which led her to believe that they were still dreaming. My questions are:
What exactly did Cobb do to Mal's totem?
How did this action directly influence Mal's belief that they were still in a dream?
Mal and Cobb were experimenting with entering deeper and deeper dream states and accidentally became entrapped in something that they dubbed 'limbo', which Arthur describes as
"unconstructed dream space".
Cobb largely accepted their situation but Mal became increasingly distraught about being trapped in a dream, eventually entering a deep and abiding depression. She came up with a solution. She 'incepted' herself into believing that their limbo reality was the real world.
COBB: She accepted it. At some point...
INSERT CUT:
INT. MAL'S CHILDHOOD HOME - DAY
Mal opens a DOLL'S HOUSE. Inside is a SAFE. She opens it- it is empty. She pulls out her SPINNING TOP.
COBB (V.O.) ...she'd decided to forget that our world wasn't real.
Mal places the top inside the safe. LOCKS IT AWAY... INT. OFFICE, WAREHOUSE - CONTINUOUS
Cobb came to believe (correctly) that killing themselves was the only way to get out and tried to convince Mal that they needed to escape. She refused, as she accepted the limbo as her reality.
Cobb was forced to incept Mal into believing that the world was not real and that they needed to commit suicide, not realising that the corollary was that when they escaped back into reality that she would continue with this delusion.
COBB: I never thought that the idea I'd planted would grow in her mind like a cancer. That even after we woke...
INSERT CUT: Cobb looks around the HOTEL SUITE, confused. He moves to the CURTAINS... Mal is on the ledge opposite.
COBB: You'd continue to believe that your world was not real...
The totem represents her belief in the reality of her situation. A stopped totem means that she's in the real world. A continuously spinning totem means she's still in a dream.
While parts of the answer by @Valorum are credible, my interpretation is simpler.
The starting point is that Mal and Cobb had both been exploring deeper and deeper dream states. They had been building their own world in the dream state and it was something they were happy with. They spent a long time there. Mal was so happy she hid her token (the token is put in the safe) so she was no longer reminded that this was a dream state (the spinning top appears to be Mal's token, not Cobb's, at least originally. Though there is another theory that Cobb's real token is his wedding ring).
The key point appears to be that Cobb recognises that they both need to return to the real world, however happy they are in the dream world, but can't persuade Mal to return. This is why he tries inception: to (re)instill the idea that the world they are in is not real. This works and they do return to the real world.
But the problem with inception is that it sticks. Even in the real world Mal still believes in the idea. Hence her suicide.
In short, Cobb did not do anything to Mal's token to encourage her to believe the dream world was real. More like the opposite. She chose to believe the world was real because she was happy in it and hid her own token to conceal the evidence otherwise. Mal was not distraught about living in the dream world, the problem was she was too happy in it to want to leave. Cobb's action was to recover the token and to initiate the first inception on her to persuade her the world was not real (it is never stated or shown but the implication is that recovery of the token didn't persuade her).
As for the tokens and who owns them. Nolan–as is his habit in other movies–leaves a great deal of ambiguity so viewers can inject their own ideas. The spinning top is initially presented as Cobb's token. But, in some of the flashbacks to the backstory, it seems to be Mal's. But, after this, Cobb seems to use it as his. Hence the various theories about his real token. But whichever theory is correct, there is a clear implication in the dream world backstory that Mal hid her token because she was happy in that world and didn't want to be reminded the world was not real. This is what motivated Cobb to try inception, precipitating the remaining events driving the movie. But there is plenty of room for interpretation here leaving a far more interesting burden on the viewer and a wonderfully ambiguous ending that makes for a far better movie than a clear explanation would have done.
