Cornelli is the term used to describe a series of non-uniform curls and turns.
In the Cake Decorating industry, it is possible to use any icing medium - even chocolate, however royal icing is the most usual choice.
When using fine piping tubes or nozzles, it is imperative that when making your royal icing, you sift the icing sugar with a very fine sieve - at least once, preferably twice - to remove any lumps or hard pieces of icing sugar. If these are included in the royal icing, any piping that you do may be impeded by a clogged or blocked nozzle.
Look at the picture above, and this is basically the design for Cornelli. I like to use a very fine tube - a #1 or even #0, and using the smallest icing bag, I place about half a cup of royal icing in the bag at a time.
I hold the bag in the palm of my hand, and use the tube much like a pencil - while applying pressure from the palm of my hand to squeeze a continuous flow of icing to the surface I am working on.
Watch my video demonstration below, to see my technique. I have designed a simple, yet elegant Christmas cake this year - and the cornelli is simply beautiful for this purpose.