The first item I ever knitted was a piece of intarsia. Of course I didn't know the term. I was 13, it was the 70's, and I wanted the brightly-designed poncho I found in a magazine. I asked my mom if I could knit it. She was pleased that I was interested in her craft, and she encouraged me. She helped me make plastic bobbins, and I wound up the different colors, letting the bobbins hang from the huge piece of knitting.
The directions said, as they always do, to twist the new color under the old, but honestly, I have never been able to figure that out. Sometimes it seems right to twist one direction and sometimes another ~ and when you are stretching a strand across a long patch of another color, you have to twist twice, so it really doesn't matter which direction you twist. I quickly learned to take advantage of this fact and change directions whenever possible in order to minimize tangle yarns. Knitting is fun ~ but untangling threads is not.
Momma Mindy tells how threads in poor storage can get tangled to an almost hopeless mass of confusion. Threads being knit into a beautiful design get tangled, too. There's no help for it but to stop once in a while and untangle them as you go. But you can also reduce the tangles by switching the direction of your twist every now and again.