This is a very “handy” technique to help you see where the best ambient light is, sometimes its obvious, other times (like when you are surrounded by white walls- it isnt’).
There are many ways to do this, I like to look at the faces of my subjects to see it and if you are not careful it is really easy to miss.
If you are ever in doubt, take a picture of your hand (which is about the size of a person’s head, unless you are taking a picture of me, as my head is unusually large), turn a little (in this case 90 degrees) and take another picture of your hand. This obviously works best in Manual mode, but with practice you will be able to see it just looking at your subjects face.
Look carefully at my hand in each shot…what do you see?
Which position would be the best to shoot a portrait based on light quality alone?