Head like glass doesn't mean your head is fragile, like glass. It means your head is clear like glass and there is nothing in it for your opponent to see. Meaning, there is no thought or pre-determined idea of what you are going to do. Your thoughts and intentions cannot be seen because you have none. Your mind is empty of any and all predetermined technique and you flow with what comes to you from your opponent.
We are taught that mind set as well, but I have never thought of it in relation to the head is glass. Nice.