To work some particularly awesome cable designs (like this one for instance), you need to learn a technique known as closed loop cabling.
This isn't something I've heard of til recently (I haven't done many complex cable patterns), but it makes sense if you think about it: the basic cable resembles a rope or braid, which can go up or to the side, but not merge with another strand. Closed loop cabling allows your strands to meet.
In order to do this from the bottom (making a cable that starts from a U shape), you have to increase multiple stitches at the point where the strands meet. Likewise to do so at the top, you need to decrease.
There are a number of techniques to do these increases or decreases, which involve a number of stitches. Here are a few tutorials I've found:
Ariadne Knits: Mirrored double increase
Ariadne Knits: Slipped decrease
Another tutorial on the Alice Starmore increase
Our Curious Home: Christmas tree increases
Our Curious Home: Cable Capping decrease