Chiasmus is an ancient literary structure taking many forms but is essentially a passage in which the first half is then reversed in the second half (E.g. A, B, B, A).
Chiasmus is found throughout ancient writing including the Bible and first recognized in the Book of Mormon by John W. Welch in 1967. While chiasmus was known at the time the Book of Mormon was published there is no evidence it was known in America (and Joseph Smith) at the time. Even if it was known, how was Joseph able to create long and complex chiasmus while dictating? It is one thing to compose written chiasmus, but quite another to dictate it.
One fairly simple example of chiasmus in the Book of Mormon is that of 1 Nephi 1:1-3
A. yea, having had a great knowledge of the goodness and the mysteries of God,
B. therefore I make a record of my proceedings in my days.
C. Yea, I make a record in the language of my father,
D. which consists of the learning of the Jews
C. and the language of the Egyptians.
B. And I know that the record which I make is true; and I make it with mine own hand;
A. and I make it according to my knowledge.
Other examples are much more complex including some 6 levels deep. How did Joseph know about chiasmus and effectively compose it on the fly?