@BartG95 Thanks! Good question: It is a traditional art technique (looking to your drawing in a mirror, software like Krita have the "M" key for that).
In art, it's easy to become blind to the error after 1 or 2h with the eyes on the same canvas. The brain tends to fix them for us. But if you mirror the artwork; the brain suddently doesn't recognize the canvas and loose its autofix ability. You suddently recover all critical cognitive perception; and you can spot the error. A precious tool:)