Knitting Help - Knitting Double Stranded

What does it mean when a pattern tells you to hold the yarn double stranded? I explain that in this video, and go a bit deeper - reasons to knit double stranded, winding the yarn together, and more. The clog/slipper pattern I mention in the video can be found here: http://www.ravelry.com/patterns/library/felted-clogs-ac33 The pattern I used in my swatch is Mock Rib: https://youtu.be/3JgmaL9tKH4 The yarn I'm using is Lion Brand LB Collection Cotton Bamboo: http://www.lionbrand.com/yarns/lbCollectionCottonBamboo.html The needles I use are Knitter's Pride Bamboo DPNs. The nail polish I'm wearing is by Julep, color "Amanda". http://refer.julep.com/s/stacip My silver ring is actually a knitting needle gauge, and can be found here: http://malojos.com/