The poster is a geometric simplification of the value structure of your image. Each form is built up of myriad gradations, which can be difficult to comprehend. The poster simplifies these gradations made up of hundreds or thousands of steps of value into a much smaller and more manageable number of steps. How many steps is up to the artist, and can be quite flexible depending on subject matter and the technique being used.
The idea is to ignore the gradations at first and instead focus on the patterns that the gradations are making. Is it a smooth transition from light to dark? Does it get gradually darker most of the way and then suddenly shift more quickly at the end? Does it shift from dark to light and back to dark? How do all these shifts correspond to the contour?
Thinking of what the gradations are doing and putting them down in steps allows us to analyze the progression of values without worrying about the technique of making the smooth gradations. There are all sorts of ways to make the gradations later, but it's important to understand what's happening in them first.