Behind the head whilst standing is a flagrant foul. But I have seen countless scenarios where 1 fighter is crouched down so low where his head is about even with the other fighter's thighs, and he gets slammed square in the back of the skull with a downward punch. If he keeps his goddam head way down there like that, is it permitted that the other fighter can chop down right onto the back of his skull? It NEVER gets a warning as far as I have seen. He is giving no other target than the back of his head.