I searched and found nothing on this, might be classed as common knowledge.
When waiting for a patch to download you get a small amount of time where you are connected directly to the blizzard server but after 10 minutes or so they stop feeding the patch to you and you're left to download from other people which from my experience takes a long ass time.
Anyway all you do is when you notice you're no longer downloading from blizzard simply exit the downloader and restart it and you get download from blizzard again