Here is the full koan:
Master Hsiang-yen said, "It is as though you were up in a tree, hanging from a branch with your teeth. Your hands and feet can't touch any branch. Someone appears beneath the tree and asks, `What is the meaning of Zen?’ If you do not answer, you break your vow to help save all beings. If you do answer, you lose your life. At such a time, how should you answer?
This is the fifth koan in The Gateless Gate collection. First, take the situation literally. You are hanging by your teeth from a tree and the final question implies that you will answer the person seeking help somehow! Be prepared to show how you would answer. Next, take the situation metaphorically, and consider whether you are hanging on by your teeth to some treasured hope or dream. In that context, ask yourself what life you will lose if you choose to let go. (This last perspective courtesy of my Dharma brother, Bill Krumbein.)