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They're called calming signals, and the reason they're calming is because they make it obvious that the individual isn't "on alert".

If the individual means you harm, they're going to be paying close attention to your every movement since they're about to attack. They'll be staring directly at you, sizing you up, preparing to pounce.

Looking away, smelling around, yawning, slow blinking or eye closing, are all boondoggles that signal, as you said, "hey, I feel at ease with you enough to do this idle busywork, almost as though you weren't here at all because I don't see you as a threat or target".