It was a great trick that took me out of the painting that I had been doing from my "mind's eye". I saw it for the light and dark areas that is was. It allowed me to see how I could rebalance it (regarding the lights and shadows). Very cool.
If you have ever seen a professional digital artist draw, you'll notice them flip the canvas repeatedly. You might think they're crazy how much they flip it, but this really does help turn off that part of the brain that got too used to the drawing. Too used to the auto correction magic our brains love performing.
This technique forces them to see that the curve they just drew feels off, even when their brain with the image right side up refused to see it.
I work in VFX and have found that after watching a shot on loop a few hundred times (over weeks) it can be quite helpful to watch it 'flopped' (horizontal mirror), which seems to be able to give nearly a whole-new perspective (on the effects-content) in many cases.
https://www.amazon.com/Drawing-Right-Side-Brain-Definitive/d...
It's a common technique when learning how to recognize lines, shapes, and shadows. The simple act of flipping an image upside down is enough for your brain to turn off it's automatic recognition magic that keeps you from seeing primary forms.
Do it enough times, and like other visual artists, you start admiring a lot of things people take for granted about vision.