The defectors would not have risked, if only they had the opportunity to leave the country legally, simply by buying a train ticket, for example. Alas, the border crossing requires a special permit, and not anyone can receive it. The ban is also imposed on the movement inside the country - in order to go to visit relatives in another village or city, people must obtain this document with a seal.
However, with some courage and a bit of luck, defectors can pass the checkpoint.
To do so, they must show the above-mentioned permission to cross the border. Where would it come from? You can guess how it happens: specially trained people take special paper, glue, tape, ink, and the artist magic turns this whole set into official papers with solid seals.
Even experienced border guards can not distinguish them from the real ones, the main thing for defectors is to keep themselves confident.