RSVP stands for rapid serial visual presentation. Words appear one at a time, in the same place, at a speed you set. It is old — the technique goes back to reading research in the 1970s, long before any app existed.
Normal reading is not smooth. Your eyes jump in short movements called saccades, pausing briefly between them. Those jumps cost time, and every time you lose your place you pay for them again.
RSVP removes the jumps. The word comes to a fixed point instead of your eye going to the word. That is the entire mechanism, and it is worth being precise about it, because most of what gets claimed on top of it does not hold.
Readers do not identify a word from its first letter. There is a spot, slightly left of the middle for most words, where recognition is fastest. Aligning every word at that spot means your gaze never has to re-centre. Without it, longer words drift and the effect breaks down.
A related and better-supported finding: display time should depend on word length. In one study of readers with central vision loss, varying the duration by word length improved reading rate by about a third. A fixed duration per word treats “a” and “extraordinarily” as equal, which they are not.
Comprehension holds up to about 350 words per minute and drops measurably between 400 and 450. That is the honest ceiling. Past it you are trading understanding for a number.
There is also a comparison that RSVP loses. When researchers compared presentation formats, showing more context at once scored better for comprehension than one word at a time — a moving highlight over ordinary text beat single-word presentation. RSVP is not the best format for understanding. It is a good format for attention, which is a different problem.
Average reading rates differ a lot by language — roughly 228 words per minute in English, 218 in Spanish, 202 in Dutch, 158 in Chinese, 138 in Arabic. Much of that gap comes from word length, not from readers. An app that offers a single global default is wrong in most languages by construction.
If you read a paragraph and realise you took none of it in, and you have to start again — RSVP addresses that directly, because there is nowhere for your gaze to wander. If you are trying to get through dense technical material faster without losing the argument, it will not help, and the research says so.
We go into the evidence in more detail in does speed reading hurt comprehension, into the alignment itself in Spritz, ORP and the optimal recognition point, and into the per-language numbers in how fast can you actually read. If you came here from the Kindle side, this page covers what happened to Word Runner.