Most file formats start from the beginning of the file, but rars are weird and look at the tail end first. Thus by concatenating the jpg and rar you have a file that’s readable as a jpg from the beginning or readable as a rar from the end. All that matters is what you open it with, affecting where you start reading the file from.

But again, anyone wise to this trick can simply spot the unusually large filesize for a jpg and figure it out. This isn’t a good method of securing sensitive files at all.