In her book, Ann Feitelson simply says try not to let the "floats" get too long.
Floats are the yarns on the inside of a Fair Isle sweater that result from carrying the yarn across the inside when you are knitting with another color.
When you have a long run of the background color, there will be a float of the contrast color on the back.
If your pattern really is a traditional Fair Isle pattern, or modeled on that sort of design, you might have floats as long as 7 stitches, a whole inch. Floats can get caught on buttons, if you wear a button-down shirt or buttoned work shirt under your Fair Isle sweater.
To prevent long floats, you can weave the yarn in on the inside of the sweater. It might show through the front a little, but not enough to disturb the pattern. When you practice real motifs, you might notice this but not so much with real Shetland wool.
At any rate, watch the second part of the video and see how Ann does an X -X -O section.
Basically, what she does is knit the first background stitch.
Then when she starts to knit the second background stitch, she puts the point of the right hand needle through the stitch.
She runs the point behind the contrast color.
She loops the background yarn around the needle point.
Then she pulls the loop of background color through the stitch, trapping the contrast color but not pulling it through.
Then she makes a stitch from the contrast color.
Do a round of X - X - O and then on the next round, move it over one stitch as you did on the X - O row. When you were alternating stitches, you got a checkerboard pattern.
When you shift an X - X - O pattern by one stitch, you wind up with diagonal stripes.
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