This is a very versatile technique in which you use a special tool (a gripfid) to split the plies of a yarn, and then feed other yarns back through. You can do some pretty amazing colorwork and stitch patterns with it. Everything I’ve made so far has this very pleasant and unusual “squishiness” to it. Most commonly it’s done with 4-ply yarns, splitting them in half each time, but there are tons of other variations. Linda Hendrickson’s books on the topic have been a huge help!