In short – shift stitch is an extra stitch(s) you work at the end of a round to make your work look symmetrical (if you thread marker moves right), or unravel a stitch(s) at the end of a round (if you thread marker moves left).
Shift stitch was made twice to keep the marker in straight line.
Without those shift stitches marker would go like this:
Red line – how marker would go without shift stitches.
Yellow line – how marker would go after first shift stitch.
Green line - how marker would go after second shift stitch.
Every time you work shift stitch, beginning of round moves to the center of the work (dotted line).
More detailed explanation.
As crochet work tends to twist to the right or left, sometimes you need to adjust position of the round marker (the beginning of the round). When you finish the current round and notice that marker moves right, you need to work one stitch into the first st of the next round and move the round marker behind the last worked stitch. So the 2nd st of the next round becomes the first now.
If you notice that the marker moves left, you need to unravel one stitch (or more if needed) and again move the round marker behind the last worked stitch. This is a shift stitch. As a rule, it is not included in the description of rounds.
Please note that it is not an increase. You do not increase or decrease the amount of stitches. The stitch count remains the same.
In some patterns it is clearly stated “work 1 shift stitch now”. In other patterns it is not included in the description of rounds but stated that you have to keep work symmetrical or keep the marker straight.
However, the amount of shift stitches and the necessity of working them depend on the yarn you use, the size of the crochet hook and your gauge. If you use some other yarn that differs from the yarn that the designer uses in the pattern, try to follow the designer’s round marker in the pictures making shift stitches (working extra or unravelling a stitch or more at the end of the round) if needed.
Why do I need to work shift stitches?
A shift st is needed to keep the piece symmetrical. Some toys have cheeks, knees, elbows and other “bumps” and “sockets”. As your work tends to twist when working in a spiral, some of these parts may go in the wrong place or misshapen. You don’t want a horse with a knee sticking out the wrong way or a cat with his head twisted at an unnatural angle. That is why you need to work shift stitches in some patterns when the symmetry matters (usually toys). Shift stitches keep the marker in a straight line instead of running around your work.
In the next picture it is clearly seen that you have to keep the marker straight. Otherwise, you will end up with ears misplaced or the snout misshapen.
Here is the example of how a toy looks like when shift stitches are not used.
It is completely twisted. Head is on the back and back side is on the front (however, it is meant to be like this in Lady pattern).
How do I know I need to work a shift stitch?
In all LOH patterns where you have to work shift stitches, it is clearly stated at the beginning of the pattern: “Keep work symmetrical and work shift stitches where needed”, “Keep parts symmetrical” or “The marker goes down the center front of the piece from the top of the head, through the center of the nose and down through the belly”.
Usually only patterns by Svetlana Pertseva and Tatiana Chirkova call for shift stitches. Other LOH patterns mostly designed in spirals where marker doesn’t have to go in a straight line.
How do I adjust the line of the round marker correctly?
Crochet certain amounts of rounds and fold the piece in half. Lay the marker straight along the folding line. Use this straight line as guidance as you continue.
Continue working following the pattern until you notice that the marker has moved aside. Do not work any shift sts until the marker has moved 1 st aside (not less!). The amount of the worked rounds may differ from 3 to 7 (it depends on your gauge, the yarn you use and the sts the round calls for).
Work a shift stitch and move your round marker behind the last worked stitch. Continue working. Repeat the steps above when you notice that the round marker has moved again. Usually you have to do shift stitches every 3-5 rounds (depending on the yarn, hook and your gauge). To check if it is time to work a shift stitch, again fold your work in half and check if the marker still goes straight.
In conclusion, even if it is said you need to make a shift stitch, it doesn’t mean that you actually will. It’s about reading your crochet stitches and being aware of how the round marker needs to go.