Two lines of chain stitch are worked side by side and then the middle edges of the 'chain links' are whipped together with another thread. It creates a beautiful, decorative and very distinct line of stitching. Sharon's tutorial for it can be found here.
I added a row to my sampler using pink cotton perle thread (8) for the base chain stitches and blue cotton perle thread (8) as the whipping thread.
You can compare the difference between the whipped double chain stitch and whipped chain stitch (just a single line of chain stitches) which is just a little further to the right in the photograph above. Don't forget to pop across to Sharon's blog and read the series of interviews she is holding with some of the talented participants of the TAST Challenge to celebrate the 10th Anniversary of TAST. You can find the latest interview post here.