When the speaker sees bent birch trees, he likes to think that they are bent because boys have been “swinging” them. He knows that they are, in fact, bent by ice storms. Yet he prefers his vision of a boy climbing a tree carefully and then swinging at the tree’s crest to the ground. He used to do this himself and dreams of going back to those days. He likens birch swinging to getting “away from the earth awhile” and then coming back.