How is the idea of "Don't wear glasses" used for myopia control?

Just as it says, the idea is that glasses are the problem so you shouldn't wear them. As your eyes become blurred, you just have blurred vision.

How does "Don't wear glasses" work for myopia control?

The shortest answer is that it doesn't work. There are millions of myopic people in the world significantly handicapped because they don't have any glasses.

Of all the methods espoused by alternative vision care advocates, the idea of throwing away your glasses has to be near the top of recommended treatment lists. The web is full of such suggestions and you can read about it in the Treatments menu under Pseudo-Science. It conjures up such a "feel-good" experience that it is hard for many to resist - just throw away your glasses (and all those doctors who falsely tried to help), maybe throw in some "exercises", get used to the blurred vision and your eyes will heal themselves. Nice image perhaps, but a false one.

Everyone became nearsighted before they got their glasses. If no glasses was a cure, there should be no blurred vision to begin with. But people do develop blurred vision due to myopia. The goal of scientists is to determine why that happens, prevent, slow or stop the process and to give people clear vision in the meantime.

Clear vision protects against myopia

Animals (including people) become nearsighted if they do not have clear vision as they are growing up. An example would be a child born with a cataract, which does not allow clear vision. If the cataract is not removed early, the eye will quickly become very myopic (and still blind). But it has been shown that brief periods of clear vision, as short as one hour a day in monkeys, are able to greatly negate the myopic effects. A corralary in humans may be that full distance correction needs to be worn at least part of every day to prevent myopic progression. If you don't have any glasses, you never have the clear vision that appears protective.

It is true that regular glasses do not help myopia if you define "help" as "make it go away". Normal glasses only put the eye back in focus for distance objects, like re-focusing a camera for a sharper picture. That doesn't "cure" the eye. It is also true that wearing regular glasses makes the condition of myopia progress faster than specialized lens corrections such as ortho-k. The question of whether not wearing glasses at all slows myopia progression has not been scientifically studied in humans because it would entail telling some people (students) that they must put up with blurred vision during the course of the study, a dubious handicap to artificially create. One study that was done with under-correction was stopped after two years when it became obvious that the under-corrected students were progressing faster than the fully corrected students. There is also the fact already mentioned that people become nearsighted before they start wearing glasses so that suggesting not wearing glasses as a "cure" is logically suspect. Clearly the situation does not have a simple explanation.

People can get used to blurred vision and they can become better at guessing at blurred letters. For some that is enough "proof" that their eyes improved. Scientists measure myopia by where the light focuses in the eye and the length of the eyeball. Identifying letters on an eye chart is a secondary method that is quite variable between different patients with normal vision. For that reason many myopia studies do not include subjective responses of clarity from patients, especially from small children.

Are there any side effects?

Besides the risk that it won't work for myopia control, your eyes will be blurred for distance vision. There doesn't appear to be any harm physically to the eye except that ignoring proven techniques prevents actual treatment. Don't drive if your vision is blurred and I wouldn't go hiking in grizzly bear country either.