Five short chunks are sent, followed by a 1-second pause, followed by five large chunks. Each chunk begins with a '[' and ends with a ']'. The five large chunks are each bigger than the typical TCP message size.
Note that this won't work with IE's implementatin of XMLHttpRequest. Opera seems to be a little weird too.
For context, check out http://blog.schwink.net/2011/01/http-chunks-and-onreadystatechange.html.