Very similar to the 'head held high' signal. Holding the chin up naturally alters the angle of the head backwards, exposing the neck, which is a signal of strength, resilience and pride. A pronounced raised chin does other interesting things to the body too - it tends to lift the sternum , which draws in air, puffing out the chest, and it widens the shoulders. These combined effects make the person stand bigger. An exposed neck is also a sign of confidence. 'Chin up' is for these reasons a long-standing expression used to encourage someone to be brave.