Tether started with a small experiment: would keeping the current word in one place make an article easier to finish on a phone? The first build took a link, extracted the article text, and kept the same reading position while switching between RSVP and the full page.
RSVP isn't a speed-reading trick, and Tether does not present it as one. The research is mixed. Showing words rapidly in a fixed position can reduce eye movement, but it can also hurt comprehension when the interface makes it difficult to pause or reread. This paper on RSVP interfaces is a useful overview.
That's why Tether uses a hold gesture instead of a play/pause toggle. Hold the screen to keep reading and lift your finger to stop. When paused, the nearby words reappear and you can scrub backward. A swipe returns you to the full article.
The idea is worth pursuing because reading and attention really are related. In one study of struggling readers, attention measures helped explain differences in reading comprehension and predicted how comprehension changed over time. Tether isn't a treatment or a diagnostic tool. It's just a reader with fewer things competing for your eyes. Read the study.
The app should make a page easier to read. It should not tell you how your brain works.
Some of Tether's settings are experiments. It includes fonts such as Atkinson Hyperlegible, which was designed to make similar characters easier to tell apart. It also includes optional bionic-style prefix emphasis, though recent eye tracking research on Bionic Reading did not find a benefit for typical readers. The setting is included without a promise that it will make anyone read better.
The same goes for tint overlays. Some people prefer reading against a colored background, but the research on colored overlays and reading is unsettled. In Tether, the tints are simply optional colors.
This is still experimental software. If a setting helps, use it. If it doesn't, turn it off and read the article normally.