How does tornado.web.RequestHandler.flush work?

Question

Python

The server looks like this:

from tornado import ioloop, web, gen


class MainHandler(web.RequestHandler):
    @gen.coroutine
    def get(self):
        self.write('1' * 1024)
        self.flush(callback=(yield gen.Callback('flush')))
        yield gen.Wait('flush')
        print 'First chunk sent'


        self.write('2' * 1024)
        self.flush(callback=(yield gen.Callback('flush')))
        yield gen.Wait('flush')
        print 'Second chunk sent'
        self.finish()


application = web.Application([
    (r"/", MainHandler),
])


if __name__ == "__main__":
    application.listen(8000)
    ioloop.IOLoop.instance().start()

Client like this:

import urllib2
urllib2.urlopen('http://127.0.0.1:8000/').close()

After the request in the server console I see:

First chunk sent
Second chunk sent

I wish I didn't see anything there. Where am I wrong?
UP: @rSedoy explained the situation, details in the comments