lots and lots of icing practice.She probably used hot water and a spatula.
there is a corner edger that someone posted about last week that looks like it would do that for you. I would assume searching for edger would pull of the posts about it.
http://www.cakecentral.com/cake-decorating-ftopict-301633-.html here is a link to the post about the edger
Or could also be the use of the upside-down method: http://www.cakecentral.com/article6-Upside-Down-Icing-Technique-for-Perfectly-Smooth-Icing.html
My guess is that is either Italian or Swiss meringue buttercream. It goes on much smoother than standard buttercream and it is fairly easy to get crisp edges like that. Here is one I iced in IMBC. After smoothing as much as possible, put the cake in the freezer for about 10 minutes and the icing hardens like butter, then you can use a chocolate scraper or a spatula with a flexible blade to finsh the smoothing. It works well.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=551113
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