This concept was based from Daniel Kahneman’s book “Thinking, Fast and Slow” that tackles on two modes of thought: Fast Thinking and Slow Thinking.
In fast thinking, our thoughts are based on our experiences, skills, senses, common perception on something, so the cognitive power that we use is a lot lesser.
Fast thinking allows us to formulate quick judgements on something, decisions, or reactions.
A good example is starting a laptop. Most modern laptops put their power button at the top-right of the keyboard. I don’t have to think or find where this button is, when I unfold the laptop, my mind will immediately proceeds to push the top-right button.
When I hear a loud sound, maybe someone is calling my name, from my right ear, my instinct will be to look at my right side (to see who’s calling me).
We are also familiar on what an “angry face” looks like, our emotional response to that is also fast thinking.