The act of capturing the essence is something I remember and love from figure drawing classes. In a figure drawing class, the 1 minute and 3 minute pose are warm up exercises you do at the beginning of a session, where the purpose is to get you to loosen up and freely and loosely capture the figure, usually on newsprint, and usually as a way to get you ready to begin the real work. With speed and brevity in mind, one tends to look more at the whole form and the bigger picture and capture the character, gesture, and energy efficiently in as few lines as possible. In 1-3 minutes there's no time to focus on details.
The figure drawing sketch epitomizes for me what the quick thumbnail sketch is about. Capturing the essence when I'm exploring ideas for the first time and then forcing myself to move on is a point that I try to remind myself of every time I approach a sketch book. When you're working fast you need to do only what's necessary to express the idea and keep moving on to the next idea.
In the beginning of each iteration of idea generation we're concerned with volume. We're concerned with trying to get as many ideas out as possible. We'll come back to refine when we select the ideas with merit, and to refine further in the next round of iterations.