The speaker is remembering a teacher whose face he cannot stop seeing as a kind of trap. The image of a rabbit with its paw caught in steel at the base of a tree suggests a man held tight by his own inner conflicts, his "contraries." His sternness reminds the speaker of his father, and the poem suggests a link to the mother too, who seems caught in the same struggle. The reference to Guernica, with its raised torch, gives the teacher's head a haunted, anguished quality when it appears over the speaker's shoulder as he writes.
Yet the feeling is not simply fear. The speaker values the teacher's "lacunae," the gaps and missing pieces, and finds an order in him, like pebbles raked around a rock. That hidden stillness gives the teacher power, and the speaker seems grateful: being tested by him made it possible to understand what he saw. The mood is tense and strange, but ends in quiet, hard-won respect.