Mirror text has a much older history than most of the novelty text tricks online today — Leonardo da Vinci famously wrote most of his personal notebooks this way, and historians still debate whether it was to protect his ideas, a natural habit as a left-hander avoiding smudged ink, or simply a preference. The technique itself is straightforward: reflect the text horizontally, the way a mirror reflects anything in front of it.
It's easy to conflate mirror text with upside-down text since both look "backward" at a glance, but they're built differently. Upside-down text rotates each letter a half-turn, the way a page looks if you spin it around. Mirror text instead reflects each letter left-to-right, the way your reflection looks if you stand in front of a mirror — your right hand appears on your reflection's left. A "b" mirrored looks like a "d"; a "b" rotated upside down looks like a "q". They can look similar for some letters and completely different for others, which is why testing the actual word matters more than trusting the general idea.
Unicode — the character standard every flipped-text tool relies on — was never designed with a full set of true mirror-image letters. A handful of characters have a genuinely close match (capital R mirrors convincingly into Я, for instance), but many letters simply don't have a proper mirrored twin in the character set, so a mirror-text generator has to leave those letters unchanged or substitute an approximation. This is a real constraint of the underlying character set, not a flaw specific to any one tool — every mirror-text generator online runs into the same wall.
The fun of mirror text is holding it up to a real mirror and watching it resolve into readable words. Generate your line, then either hold your phone screen up to a mirror, or hold a printed version up to one — a bathroom mirror or a compact mirror both work fine. Because of the Unicode limitation above, don't expect perfect resolution on every letter; short words and names tend to read best, since there's less room for an imperfect letter to break the illusion.