How do Cobb and Ariadne enter Limbo without having to die?

Question

After Saito dies on the third dream level, he enters limbo. Ariadne then convinces Cobb that it is necessary for them to enter limbo to go after Saito, in order to pull him out. They do so by hooking themselves up to a dream-machine on the third level, within the snow fortress, and use this to enter limbo.

How are they able to enter limbo in this way? It had been previously explained that to die in a sedative-assisted dream would cause one's subconscious to enter limbo (and this is in fact what happened to Saito). But wouldn't using a dream machine on the third-level just cause Cobb and Ariadne to enter a fourth level? Why do they end up in limbo via this mechanism?

Answer 1

The sedative compound that Yusef creates for the job is specifically designed to keep the dream stable for three levels.

COBB: We need you there to tailor compounds to our particular requirements.

YUSUF: Which are?

COBB: Great depth.

YUSUF: A dream within a dream? Two levels?

COBB: Three.

YUSUF: Not possible. That many dreams within dreams would be too unstable.

COBB: I've done it before. You just have to add a sedative.

This means that the fourth level and beyond are still unstable. We know from other dreams in the film that when the dream becomes unstable, it collapses and the dreamers wake up. Since they are sedated, they can't wake up, so when Dom and Ariadne try to go deeper while already on the third level they instead fall into Limbo.