This is a dramatization
Our hero is sitting in her living room watching a movie and finishing up the last major knitting on an intricate 5 color Fair Isle sweater. The sleeves are complete and only a few more rows of torso are left. She's in the zone and happy. If she doesn't make any major mistakes, this cardigan will be sewn together and done tonight. The hardest question after that will be whether to wear it right away, or wait until it's been washed and blocked first. During a battle scene, she ruminates pleasantly about how well the tube she is knitting will fit her torso once she adds the sleeves. She's never made a sweater like this before, and she really enjoys how easy it is to work the pattern without dividing for arm holes.
Where are the arm holes? OH MY GOD it doesn’t have ARM HOLES!!!
How on earth is she to attach the sleeves to the sweater?! There are no armhole seams! The sweater just goes 'round and 'round with two little 1" gaps where the bottoms of the sleeves should be but last time she looked her upper arms were a lot bigger than 1" in diameter. Granted, it was really easy keeping track of the pattern, working the color changes, and maintaining even tension without having to put in arm holes, but not having armholes is a problem!
And wait... isn't this supposed to be a cardigan?
It's a tube! *cry*
*voice from off screen*
No. She shakes her head emphatically. She is not taking a pair of scissors to the tube. Her look screams "you have got to be joking." She doesn't care how many people have done this in the past, there's no way this will work. She just spent 4 weeks making this gorgeous Fair Isle pattern and she's not going to cut it and have it unravel. No. Just, no. Please, no.
This has been a dramatization of what many knitters feel when they first learn how to steek a sweater. A steek, to steek, steeking, this is when you take a pair of sharp scissors and cut an opening in your work. It's a heart stopping exercise, and requires a hefty dose of faith.
The wonderful thing about steeks is that they make colorwork easier and they often remove all the tedious weaving in of ends which plagues such knitting. And if you are making a cardigan, you can place the color change in the middle front, which eliminates the jog when you change colors while working a tube. And a well made steek is secure enough that you can simply weave in any last ends you may have (say, from crochet steeking) and wear the item as is. There’s no need to sew the steeked edges back to secure them or make a finished edge. If a button band is desired, it can be worked directly off of the steek as well. Sleeves can be sewn directly into steeked armhole openings as well.
There are two basic kinds of steeks. The Fair Isle steek and the Norwegian steek. Both are most commonly done with the intricate stranded colorwork of those two knitting traditions because those types of sweaters are much easier to do in the round and because it is easier to keep tension consistent if the entire piece is worked in the round instead of partially worked flat. Sleeves have to be added somehow, and why make cardigans harder than they need to be? In both traditions, the fabric will be cut apart to accommodate armholes, cardigan openings and even neck shapings, and all without wasted fabric. Steeks aren't just for stranded colorwork sweaters, of course. It is a simple and useful technique with any number of applications.
Fair Isle style steeking is a technique which takes advantage of the natural tendency of wool fibers to catch hold of each other and felt. Some yarns, such as those made from 100% Shetland wool, cling to themselves so readily that there is no need to secure the edges of the fabric before cutting. Fair Isle steeks are worked over extra stitches which form a seam allowance for attaching sleeves and such. This is not a technique to use on yarns which are slippery and superwash wools should only be used if you use an anchoring method as well. For the faint of heart, the edges can be secured by hand or machine stitching (hand stitching is superior). Another method is to secure the stitches with a line of crochet stitches. I prefer this method as it is easy, elastic, and does not require me to dig out my sewing machine or find matching thread. Also, using the same grabby yarn means that the crochet steek gets stronger as it is worn.
Norwegian steeks are not worked over a seam allowance and the body of the sweater is worked up very differently. I find the sweater shape far less intuitive than Fair Isle steeking so I doubt I will ever explore that method since the Fair Isle technique can easily be translated to other sweaters. Because the Norwegian steek isn’t worked with any seam allowance, the edges of the steeks must be secured before cutting so there is a reinforced area for stitching. This is generally done by hand or machine stitching (hand stitching is superior).
The one caveat to the "fewer ends to weave in" benefit of steeking is a method which is suitable for slippery yarns, but it eliminates all the joy and ease of the other steeks. In this method, you leave a long loop on every row where the steek will be cut. Then, after it is cut, you go back and weave in every blessed strand back along the row. This means 2 ends to weave in for every row cut. It just does not seem worth it to me.
For a tutorial on how to steek: See Eunny Knit!.