Theory
Working memory is a small room.
John Sweller called the traffic in that room cognitive load. Too much and nothing sticks. Too little and the hour is empty.
In 1988, in Cognitive Science, 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 gloss, a page 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 reader, today.
Tap-to-define is also load work. Looking a word up in another app is extraneous. Keeping meaning under the thumb leaves room for the sentence.
Encoding and retrieval say what the hour is for. This page says how much of it you can spend at once. The ranking is the control.