Illustration 1 shows the nature of the problem. Specifically, when two columns of stitches are to be separated, the only thing holding the fabric together under the separation is a single stand of yarn (illustrated in green). That single strand tends to stretch out, and will eventually leave a hole in this area.
Illustration 2 shows that by crossing the stitches in the row just under the separation, there will now be five strands of yarn to take the strain (green) rather than the single strand in illustration 1. (As to how to cross the stitches, the easiest way is probably to spear one stitch with a bobby pin and let it hang on the back or the front of the work, knit the next stitch, and then replace the stitch from the bobby pin onto the left needle, and then knit it. Whether you allow the bobby-pinned stitch to fall to the back or the front determines whether the front stitch of the crossed pair slants right or left)
Illustration 3 shows an application of this principle at the heel tab of a sock.
Illustration 4 shows crossed stitches at the bottom of a vertical opening such as a pocket slit or a vertical buttonhole, or at the bottom of a sleeve opening.
Illustration 5 is the same as illustration 4, but shown "in the wool." As you can see, the stitches are crossed differently in illustrations 4 and 5, and it is up to you to decide which way you like better--structurally, it makes no difference at all.
Crossing stitches makes a sturdy utility reinforcement--very good for socks, buttonholes, sleeve openings and children's clothing. However, this method makes a noticeable pucker in the fabric, and therefore is perhaps not so wonderful for a v-neck sweater, where (depending on the further edge treatment) the pucker created by crossing the stitches might be on very obvious display.
A note to knitting geeks: there is one additional application of crossing stitches which is quite lovely. When you KNOW you are going to use a Norwegian sleeve "psuedo-steek" (no additional stitches added for the steek) you can cross the stitches in the row UNDER where the cut for the sleeve steek is going to end. In other words, after you have secured the two columns of stitches on either side of the intended cut, then when you come to cut the "ladder" between the two columns, there will be a nice pair of crossed stitches at the bottom of the ladder, just waiting to take the strain at the bottom of the newly-made opening.
--TECHknitter You have been reading TECHknitting on "crossed stitch reinforcement for the bottom of a vertical opening in knitwear."