HOW TO PREVENT HOLES AT THE SIDES OF ANY HORIZONTAL OPENING
These may include underarms in top-down garments, gusseted and gussetless thumbs on mittens and gloves, forethought and plain heels on socks, etc.

Step 1a. In case of an opening (thumbs, forethought heels, pockets…): slide the required number stitches onto waste yarn, then cast on provisionally an appropriate* number of stitches to replace them, using the invisible method with the same waste yarn as auxiliary thread. You may use a separate length of waste yarn for the invisible cast-on as well. After working a couple of rownds, slightly pull and tie waste yarn ends, so that the edge stitches remain nice and tidy. Complete the detail, or work about 2” (5cm) above the opening.
If there are a lot of stitches to cast on, use the technique described in Step 1b instead of the invisible method.
* There will be 3 stitches added to the casted-on ones, so please take this into account if you need the exact number.

Step 1b. In case of a two-directional “join” (underarms, plain heels…): using the crochet chain method and waste yarn, cast on provisionally the required* number of stitches and work about 2” (5cm) above the cast-on, or complete the detail.
Important: in the first rownd, make sure to knit all the newly casted-on stitches regardless of the pattern.
* There will be 3 stitches added to the casted-on ones, so please take this into account if you need the exact number.

Step 2. Retrieve the stitches put on hold (if any) and undo the provisional cast-on, placing live stitches on the needle. Note that you will have two “half-stitches” at the very edges, and the total number of live stitches, including these half-stitches, will be 1 stitch bigger than the number of provisionally casted-on ones.

Step 3. Join the yarn and proceed to Step 4a or Step 4b accordingly.

Step 4a. Tricorner at the left edge of the live stitches.
Work to the last “half-stitch”, work it crossed to the right to close the gap. (You may need to reorient the “half-stitch”.)
Find the “main body” stitch before the retrieved stitches. Insert the crochet hook into this stitch, beneath its right leg (which may appear “left” to you since you look at it upside down), and up back to RS, then turn the hook to the right, so you have a twisted loop on the hook. Pull remaining left leg of the same “main body” stitch through the loop, then place the resulting stitch onto the left needle and work it as usual.

Step 4b. Tricorner at the right edge of the live stitches.
Work to the end of the retrieved stitches. Find the next “main body” stitch of the same rownd. Insert the crochet hook into this stitch, beneath its left leg, and down back to RS, then turn the hook to the left, so you have a twisted loop on the hook. Pull remaining right leg of the same “main body” stitch through the loop, then place the resulting stitch onto the left needle. Work it as usual, then work the next “half-stitch” crossed to the left to close the gap. (You may need to reorient the “half-stitch”).

Please note that you will end up with 3 additional stitches on the “live” side.

EXAMPLES
Underarms:
Dune
Bohus Lopi

Heel:
Multi-Stem

Russian version can be found here.
IGTV in Russian: p.1, p.2.