(The machine allows a person to share dream space with others, but since only Cobb and Mal have been there, the place is filled with things created by both of them. It's only when we wake up that we realize something was actually strange. Cobb said before: Never recreate from your memory. When a dreamer is there too long, his/her mind mind starts to think its real. It only contains things the dreamer built in it or what the dreamer intended there to be, which could explain why Limbo has so few projections. Limbo is an infinite dream place of shared subconsciousness. The result is the same as the inner ear only detects the acceleration due to a force acting on the body, much like Eames' demonstration of tipping Arthur's chair over to wake him up. Gravity alone is not enough to awaken the dreamers, but when the lift hits the roof at the top of the shaft, the sudden stopping or slowing-down causes the dreamers to first be propelled upwards, and then fall back to the floor. To those in the lift, the force of the lift pushing them is equivalent to gravity. He improvises a kick by moving everyone into the lift and then setting off an explosive to cause the lift to start moving up the shaft (this works much like a rocket on a space craft). However, the timing is thrown off and now that Arthur's body is in free-fall in the van, it translates into his dream of the hotel - thus preventing the floor from dropping once the explosives discharge. This is supposed to happen before the van plows through the barrier and off the bridge. The original plan is to have the floor of the hotel room drop out from underneath the dreamers and thus provide the "kick" that wakes them up-specifically Eames who is "hosting" the dream in the snowy mountains.