A tiger caughta fox in a forest, and was just about to eat it, when the fox said, "You mustn't eat me. I was sent by Heaven to rule the animals. By eating me, You will violate the command of Heaven. If you dont believe me, just follow me to see whether the animals are afraid of me." The tiger agreed, and followed the fox as it walked around the forest. The animals all ran away on seeing them. The tiger thought they were afraid of the fox, so he let it go. He didnt realise that it was him that hte beasts were really afraid of.
This idiom means relying on another 's power to bully or frighten others.