嘴角扬起的那一抹微笑
[zuĭ jiăo yáng qĭ de nèi yī mŏ wēi xiào]
It refers to 'the smile at the corner of the mouth'. It usually describes a subtle but significant expression, possibly hinting at positive emotion, confidence or mystery. The slight upturn of the lips can also mean someone trying hard not to laugh loudly in certain contexts.