Theory
Cognitive load.
John Sweller's 1988 paper treated working memory as a scarce resource. Too many new pieces at once and learning stops. Too few and the hour is wasted. Inlet designs for that limit.
In Cognitive Science in 1988, Sweller described problem solving as something that can crowd out learning. Working memory holds only a few novel items at once. If a task spends that space on juggling, there is none left to build the long-term structures you came for.
Later work with Jeroen van Merriënboer and Fred Paas split the traffic into kinds. Intrinsic load is the difficulty of the material itself. Extraneous load is the difficulty of the presentation: a bad layout, a hunt for a definition, a screen that makes you hold too many unknowns at once. Germane load is the useful work of actually building the schema.
Overload is when intrinsic plus extraneous fills the room. Underload is when nothing in the room is new, so germane work never starts. The useful hour is the band in between, where there is enough novelty to learn and enough spare room to do it.
Inlet Language uses an unknown-word count as a load estimate. A text with a wall of new words is overload: you spend the hour decoding and never encode a story. A text with nothing new is underload: you feel busy and the known set does not move. The readable band is an attempt at optimum load for this person, today. Looking a word up in another app is extraneous load. Keeping meaning under the thumb leaves room for the sentence.
The same limit will apply to the next Inlet tool. We will need a different sensor than unknown words. We will still have to keep the hour out of overload and out of underload.
Encoding and retrieval say what the hour is for. This page says how much of it you can spend at once. The ranking in Language is the control we shipped first.