plot explanation - Why do the kicks have to occur simultaneously? - Movies & TV

In Inception, why do the kicks on the different levels have to occur simultaneously, rather than one after another?

Answer

I love the question.
Here's a summary from a very tight explanation (with some drawings):

Inception and Kicks

There were two types of dreams. The simple construct with a mild sedative and the more complex construct with a strong sedative.

In the simple construct, one kick is enough. Kick can be either to the self (eg: Arthur shot in the head) or a kick from above (eg: Cobb dunked in bath tub). The sedative is light so there is no synchronization required. In real life this can be equated to sleeping and dreaming about falling, the fall in the dream wakes us up. Alternately someone wakes you up by pushing you off the bed.

In a strong sedative, complex construct dream - a single kick is not going to be enough. A person needs two synchronized at the same instant. One kick to the self and one from the level above. The synchronization is done using music. In the real world, this can be equated to being under total anesthesia. Neither would a bad dream fall wake you up nor would pushing you off the operating table. The reel part here is that if some how pushing you off the table can be coincided with the fall in your dream, you can wake up.

Now, in the act of inception on Fischer's mind, they are in a complex heavily sedated dream. The only viable kick here is falling because death will take you to limbo. As mentioned in another answer, waiting it out is not going to work because weeks and months would need to pass in the lower layers. Waiting it out is a possible option only in the first layer because there is limited time dilation. Also a single kick from a level above or jumping off a cliff in the current level wont work as a kick. The dreamer is far too sedated for that. A person needs to fall in the current level and the level above simultaneously so so they plan. The following kicks (falls) are planned:

Level 1 : Van Falling
Level 2 : Falling by one floor (blowing up the floor)
Level 3 (inception level) : Falling from the building (blowing up the structure)

Of course, things don't go as plan. Hence they improvise on the kicks.

Here is the set of synchronized kicks that each one receives per level of dream.
Point to note, no two people receive the same two kick pairs.

Here's the image from the link:

So, other than dying in limbo, everything else needs a pair of synchronized kicks. For more details, read the link.