preventing the skim top from forming on chocolate pudding

I was amazed and I'm sure that most of you may know this I was watching a cooking show and the lady was making chocolate pudding and she poured it in little coffee cups lined on a cookie sheet then she placed little squares of plastic wrap on top of each cup allowing the wrap to touch the top of the pudding then she put it in the refrigerator she said that prevents that skim top from forming then later she took them out they were smooth looking I was just in awe of her I never even thought of that