Close your eyes and think about a cube, what does your mind see? A Perfect Cube. 90 degrees angles, every side the same length, it is well defined. You can even make it rotate in every directions.
Now close your eyes and think about your top of the swing position, what do you see? You can probably see a top of the swing position. Do you picture it as clear as the square? Probably not.
Well a Cube is a Cube, you know what a cube looks like because it never changes. A Cube will always look like a Cube.
A swing on the other hand is always different, you see thousands of different swings in a lifetime. That is the reason why it's so hard to picture. And the reason why you need a model, a swing to look at, a swing that will be for you a truth.
Using a model, you'll be able to picture the swing positions just like you can picture a Cube in your mind.