@Wak *******
basically you get a visa exemption and before it expires pop down to the immigration office complete the documents and slap 1900 baht on the table you have an extension. Before it expires, leave or you will over stay an get a black jack on your file.
At the moment things are a bit in the air. You can leave and return and get another visa exempt. It's called a border bounce. But that is not the intention of an exempt visa . So you are at the mercy of the IO. People have been denied entry.