You are testing your game with someone. You see her not understanding what to do, having a hard time realizing that some feature is there, or just struggling to click on some object, and you feel bad. You feel like she is suffering because of you, too shy to say “I don’t understand this!” because you are there, and you want to ease her suffering. Don’t explain, wait. Unless what they are struggling with is not what you want to test, keep your mouth shut. Just tell them something: like “I will explain later, I promise, but now I want to see how you understand this. If you don’t, it is my fault, not yours“. Keep your eyes open, write down what you see they do, what they don’t understand, but try to resist the urge to talk until they finished their play session. If you start explaining you might later forget what was the thing that was not easy to understand, and you will never improve the usability or design.