How does the Bone Morph work？

I use a pmx mmd model transfromed from XNALaraXPS, and it is imported into blender(ver.2.79) well , but the character has no facial expression. And the in the Bone Morph I can see some tags like にやり左、真面目, so how do those bone morphs work? If I want to use these bone morph as facial expression, what should i do ?