Chocolate must be "tempered" for decorative work so that it sets up glossy and crisp; otherwise it would set but not be very firm, and would look dull and streaky. Chef K melted two bowls of chocolate, one dark and one white. It is melted at about 110 degrees. Unmelted chocolate pieces are added and stirred in (off the heat) until the temperature drops to the ideal zone (86-90 degrees). The goal is to end up with the chocolate at the correct temperature just as the last couple pieces melt in. These final pieces are the "seeds" that carry the proper crystalline structure. They start a chain reaction of crystalization throughout the bowl - the rest of the chocolate copies and multiplies the fine crystal pattern, so when the chocolate cools enough, it sets up as it should - with a perfectly smooth, ultrafine crystal structure. If the chocolate were not tempered properly, the whole batch would form any old size and pattern of crystals upon cooling. And then the chocolate wouldn't look pretty! But it would be just fine for re-tempering or eating.
Working on a marble slab, Chef demonstrated chocolate decorations. For white and dark chocolate "cigarettes", he spread a really thin layer of white chocolate on the surface, then drew a toothed pastry comb over to remove all but parallel lines of white chocolate. He let it set up for a minute, then spread a very thin layer of dark chocolate over the whole thing. When it was just firm enough, he used a bench scraper to cut in and scrape up perfect curls of striped chocolate.
This photo demonstrates the concept much better than my description!
Chocolate Cigarettes (unknown photo)
Chef then made glossy colored chocolate squares by spreading chocolate on printed acetate sheets, and chocolate spirals. These he made by spreading chocolate on long rectangular acetate sheets, using the toothed pastry comb to strip away chocolate, leaving 1/8 inch lines of chocolate. He rolled the acetate sheet and let the chocolate set. When peeled off, it looked a lot like what's on this cake (unknown photo, cake, etc).
Chocolate ribbon spirals