We can’t see exactly how the brain works its magic in generalizing from many examples to just knowing what structure to automatically use to express a meaning it has never tried to express before — which is what fluent speakers can do.
What if we think of it in terms of tagging?
It’s sort of like the man in the brain has to tag each structure that comes in, but to tag the structure — to sort of label what that structure would be good for in the future — he needs to have a meaning to figure out what tags can be stuck on it. The brain is matching these structures up with categories that don’t even exist yet (it’s way cool, if you think about it!) After hearing a sentence, the brain says, “Hmmm…I haven’t heard anything that does that particular thing before in the language. I’ll call that ‘ongoing action’ for now. Let’s see if I get anything else in here that also does the same thing. We might be on to something here!” And when the next sentence comes in “She is eating jellybeans,” the brain says, “Aha! Told you so! Ok, let’s confirm that ‘ongoing action’ category and see if we can’t get more examples so we can really master this thing!”
Let’s say the brain guy hears “he is walking”. If we think about a tagging model, that might be labeled as “how to talk about one person”, “how to talk about an action that is ongoing”, “how to talk about bipedal locomotion”. The same sentence helps the student acquire language in multiple areas or “categories” of function and vocabulary. But to acquire the present progressive (one particular tag that has been put on that sentence), the brain needs a whole bunch — thousands — of examples of structures that can ALSO be tagged with “how to talk about an action that is ongoing”. If the brain doesn’t know what “is walking” means, that tag cannot be applied, so that is one repetition less toward acquisition.
He is walking -> ongoing, one person, bipedal locomotion
He is walking to the store. -> ongoing, one person, bipedal locomotion, locative goal, retail outlet
He is not dancing to the store. -> ongoing, one person, happy-happy bipedal motion, locative goal, retail outlet, negative
Is he dancing to the movie theatre? -> ongoing, one person, happy-happy bipedal motion, locative goal, place-of-entertainment-with-motion-pictures, interrogative
So from a structural perspective, this circling or story-asking has gotten us these counts:
one person: 4 reps [structure]
ongoing action: 4 reps [structure]
bipedal locomotion: 2 reps [this is vocabulary, really]
happy-happy bipedal motion: 2 reps [vocab]
retail outlet: 2 reps [vocab]
place of entertainment with motion pictures: 1 rep [vocab]
interrogative: 1 rep [structure]
So this is now 4 reps toward the unknown-but-high number needed for the brain to be able to automatically express the concept of “ongoing motion” no matter what meaning is specifically being handled — he is swimming, she is laughing, they are eating, whatever.
It is possible, if we look at things this way, that someone could notch up a rep in “ongoing action” without getting the rep in one of the vocabulary items. The danger is that the brain has limited processing power (see Daniel Gile’s excellent work on the effort model in interpreting, in which he talks a lot about comprehension as it is key to the interpreting task), so it needs to devote more of its “processing resources” to figuring out unknowns. Sometimes, if too much has to be devoted to unknowns, other things get lost. (Interpreters will often miss information, even easy information, if they are being loaded with a heavy accent, unknown vocabulary, or other things that tax their processing capability.)
So to me it follows logically: making sure that all input is 100% comprehensible during input that is intended to provide repetitions toward acquisition just makes sense. Will we always reach this standard? No. We’re human. We have off days, we get distracted, whatever. But IMO it should be the goal.
Comprehension checks are often taught as an afterthought, with circling being the most important element. I have started putting two CC (for “Comprehension Check”) symbols on the Hexagon of Circling, to emphasize that even as you are randomly working that circling pattern, you should be including comprehension checks of the very direct “What did I say?” variety.