Last time we discussed short rows, and how they work conceptually. As promised, this time we're going to learn about why short rowing causes gaps to form at the turning points, and what your choices are to prevent that.
To figure out why short rowing causes gaps, let's revisit the little ASCII diagram from the last post. Once again, Xs will indicate actual stitches, and dashes the places where stitches don't exist because you turned before you got there. So, short rows look like this:
In knitted fabric, a stitch that's in the middle of a row is typically connected to four other stitches: the one above and the one below in the same column, and the one to the left and the one to the right in the same row. When you turn back for a short row, the last knitted stitch on the row does not get connected to the stitch on the other side of the turning point -- it's more like an edge stitch, connected above and below but only on one of the two sides. For instance, the 3rd-column stitch on row 5 is connected to the 3rd-column stitches of rows 4 and 6, and its right leg is connected to the 4th-column stitch on the same row, but its left leg isn't connected to anything. The 2nd-column stitch on row 4 is connected at the top to the 2nd-column stitch in row 11, but not connected to row 5 at all. You've essentially got edge stitches in the middle of your fabric. This is what causes the gap.
In order to not have a gap, you've got to connect those edge stitches to the stitches on the other side of the turning point in some way. There are a number of ways to do this. The charming Nona did a nifty short-row experiment a while back, in which she demonstrated three methods: the wrap, the yarnover, and the Japanese methods. She's done such a lovely job of covering the mechanics that I'm not going to repeat it; instead, I'll discuss why doing one of these techniques makes your gap disappear.
What you are doing when you do one of these techniques is giving yourself a place connected with the stitch on which you turned, which you can then come back and connect with the stitch on the other side of the turning point. With the wrap method, you do this by putting a little loop around the base of the stitch past the turning point; with the yarnover method, the loop goes on the needle instead, but it's essentially the same thing, assuming you were going to hide the wraps using the wrap method. With the Japanese method, what you're doing is a smidgen different, since you're just marking the working yarn and not actually drawing an extra bit of it out; it's a tad tighter, which may be why some people find it a little bit more invisible.
Let me expand for a moment on why I said the wrap method and yarnover method are "essentially the same thing." When you do the wrap method, you could actually just leave the wrap, if your sole concern were pulling the gap closed. However, the wrap is fairly obvious on stockinette, sitting there like a little collar around the base of a stitch. Therefore, it's usual to pick up and hide the wraps when you come back to the wrapped stitch. When you do this, you lift the wrap up from around the base of the wrapped stitch, and put it on the needle. At this point, you've actually opened the gap back up, but then you go ahead and work the wrap together with the stitch it was on, and that closes it again. What you've got at the moment when the wrap has been lifted up onto the needle is basically the same as what you've got when you've done the yarnover method instead, and that's why I say these are "essentially the same thing."
Most of the time, which gap-closing method you use is merely a matter of preference, based on what you personally find easier or more pleasant to do, or which gives you the nicest finished appearance. There is a circumstance, however, where the wrap method may have a clear advantage, and that's where your project is such that hiding the wraps is unnecessary. This may well be the case if your project is in garter stitch, or if your project is going to be felted; it's almost certainly going to be the case if your project is in garter stitch and going to be felted, which will render the wraps completely invisible. Under that circumstance, since you will not have to execute the rather fiddly step of hiding the wraps, the ease of executing the actual wrap will generally make this method the obvious preferable choice.
I definitely encourage you to try out the various short row methods for yourself. Repeat Nona's short-row experiment if you like, and see which suits you best.