When someone stands in their normal relaxed stand, or even stands up straight to have nice posture, there is a little bit of flab right underneath their chin. No matter how skinny they are, you will see this. If you tell people to bring their chin forward, which sounds like the sensible thing to do, they will point their chin at you, which brings their face up and ends with you shooting up their nostrils. (Not attractive.) Instead, tell your model to bring their ears forward.