It came out of research they collected and gathered from various sources that was initially focused on speeches and speechmaking - for example speeches by Winston Churchill and Gordon Brown. What works, what doesn't.
But, as we know, the power of words is the same, if used in the real world for speeches or in dialogue for scripts. So this idea shouldn't be seen as something to only use in big speeches in films, but rather in all dialogue.
It falls into two parts.
ONE - paint simple pictures in the mind's eye
If a character is talking about experiences that have effected them, about memories, about ideas, about thoughts - then abstract is not the way to go. You may think this is obvious, but it isn't always. For some writers the thought process around this issue may go something like, if it is a very specific action I will show it on screen and then if it is a slightly more abstract idea, I will have a character talk about. This research shows that even in the abstract parts, we need personal detail. Hannibal Lector can't talk about the ironic quest for personal freedom, he must talk about eating the liver of a census taker. Sure, you could show the scene, but the power of VISUAL language is stronger.
Perhaps it is because we don't follow this idea that voice overs are hated so much. They are often abstract and up in the air. Not grounded in personal, visual language that connects to us, the audience.
TWO - simpler, earlier language is better for the hero.
If you look at the speeches of Winston Churchill for example he uses the simpler words (which happen to be the old English words) for talking about the British. These are childhood, easy words. Therefore, using these words takes us right back to early experiences - fight, beach, dog.
When he talks about the enemy he uses more complex words. Words like 'mechanised, machinery, invaders'. These are harder words for the brain to process. The brain prefers the 'hero words'.
Some examples may help... Imagine the opening to a movie. A voice over accompanies images from childhood.
A version that was too abstract would be "The range of human emotions is wide. Having pleasant experiences, happy times, in your childhood will enable you to develop into a fully rounded adult"
It is okay. But what it needs is some personal images. Things we can latch onto. It can still be philosophical in nature, but it connect more easily to the audience.
Better visual dialogue would be "When I was a boy I would smile as the sun hit my face and cry as my knee scraped the surface of the rough playground. Without these, without the smile, without the tears, I would not be the man I am today".
Not the best dialogue I know, but you get the idea I hope.
Now, clearly some characters may talk differently to others. But this research shows that the bottom line will impact on the audience more and be more memorable and easier to digest.
It is early days with this idea. But it seems powerful. And simple.