For nothern climates (it's best to do this in late July - early August)
This is a website that has a presentation on overwintering nucs by Michael Palmer, he is in Vermont. His technique is working well for him. My bee business partner and I are trying it out for this year - so far so good. It all makes sense to me and those that I trust.
It's a good method for those with multiple hives to manage. It's a means to hedge your bets on getting colonies thru the winter. http://www.yulesapiaries.com/VideosandPics.html