This question is fairly self-explanatory -- watching Boult conduct is almost magical. There are some excerpts from a BBC documentary floating around that explain in part the development of is singular technique:
But did he write any other documentation on his baton technique and why he selected such a lengthy baton? Which, uh, how long was that thing? It looks like he's majestically swinging a fishing pole at the orchestra. And yet he's probably one of the best conductors I've ever seen.