The main thrust of the book was looking into the ongoing debate as to whether implicit teaching styles lead exclusively to implicit knowledge and explicit styles to explicit knowledge only, or whether explicit teaching could lead to better implicit knowledge. It's an important area of discussion because at present the mainstream theory of language teaching holds that only implicit learning can ever lead to implicit understanding and production, and that explicit teaching only ever makes people consciously aware of rules and able to apply them mechanically and consciously.
And yet there are very few teachers who don't include some explicit instruction in their lessons, whether that's word-lists or conjugation tables. (Even Assimil, who sell themselves on the principle of natural assimilation, dedicate more letters to grammatical explanations than they do to the dialogues and their transcriptions and translation.)
I haven't read the whole book yet, and (unsurprisingly) what I've seen so far is pretty inconclusive. However, it does lean towards the opinion that explicit teaching does indeed help in language mastery. It also discounts a lot of the past counter-evidence to this theory on the grounds that their models of explicit teaching are simply bad examples, using overly mechanical, rote methods, and not being equivalently "meaningful" to the implicit method under examination.
It's this word "meaningful" that I think is the crux of the problems faced in language learning – language is nothing without meaning.
In the language classroom, items that seem to be inherently rich in meaning can paradoxically be rendered devoid of meaning by context.
My cousins buy trousers.
In an objective sense, it carries a lot of meaning, and there is no truly redundant information in the sentence – every word, every morpheme, brings something not explicitly present elsewhere. (But even then, it has no real personal meaning to me, as I can't imagine myself ever saying it. This is a side issue for the moment, though.)
But what happens when we put that sentence into a classroom exercise?
For an extreme example, let's take the behaviorist idea of substitution drills. In "New Key" style teaching, a substitution drill would be target language only, and one element of the sentence would be substituted with something else in the target language. So our theoretical exercise might go:
Teacher: My aunt buys hats
Learner: My aunt buys hats
Teacher: My mother
Learner: My mother buys hats
Learner: My mother buys trousers
Teacher: My parents
Learner: My parents buy trousers
Teacher: My cousins
Learner: My cousins buy trousers
At the point of utterance, the learner does not have to pay any attention whatsoever to the meaning of anything in the sentence beyond the plural marking of my cousins and the s-free verb form of buy, which is made even easier by the fact that this plural example follows an earlier plural example. Thus the student has no immediate motivation to attend to meaning, and it is a struggle to do so.
Substitution drilling is, as I said, an extreme example, but I do feel it is useful in establishing a principle that affects a great deal of learning, even where the effects are not so obvious.
Consider, for example, the fairly established and mainstream idea of focusing on a particular grammar point in some particular lesson, or section thereof. If I am set a dozen questions all of which involve conjugating regular Spanish -er verbs into the present simple third person singular (or whatever), then I do not need to attend to the meaning of the present simple third person singular, just the form -e.
To me, attending to meaning is the single most important matter when it comes to language learning, and yet it is rarely explicitly discussed. Instead, it is typically wrapped into a specific embodiment of the principle. Krashen's comprehensible input hypothesis suggests we learn language by understanding input, the communicative approach says we learn when we use language to solve problems. Total Physical Response says language has to be tied to physical action All of these are attempts to address the meaningfulness of language, but they are a narrow, specialised form of attention to meaning. CI and TPR deny us access to the colourfulness of abstract language with its subtle, personal meanings, and the CA doesn't do much better in that regard – while modal language may be taught in a communicative classroom, the nature of the task implies a rather pragmatic, utilitarian meaning, so there isn't really any meaningful difference between blunt orders like give me it; plain requests (can I have...?) or those indirected with a conditional mood (could I have...?); and statements of desire either, whether in declarative (I want...) or indirected further in the conditional (I would like...).
Other teachers take the idea of "personalisation" and raise it above all other forms of meaningfulness, insisting that students only learn by inventing model sentences that are true for them. But isn't (eg) I want it, but I don't have it true for everyone? Does the brain not immediately personalise a sentence such as that? (When I came across a similar sentence in the Michel Thomas Spanish course, I was cast back to throwing coins in a wishing well as a child.)
Perhaps the reason few writers wish to discuss attention to meaning is that it throws up a lot of questions that often fundamentally challenge their methodologies. For example, comprehensible input (and any similar learn-by-absorption philosophy) is confounded by redundancy in language – there is no need to attend to the meaning of every morpheme when the same information is encoded twice in the sentence. Perhaps the clearest example of this is the present tense -s suffix for English verbs (third person singular, i.e. he/she/it). It is readily apparent that a lot of learners do not pick this up, and there are a great many foreigners who spend years in English speaking countries, hearing thousands of hours of input with the correct form, but who never pick it up. There is no need for the learner to attend to the meaning of the -s, because they already know from the subject of the sentence that it's the third person singular being discussed. When they speak, they are understood, because even though it sounds incorrect to a native speaker, there's practically no risk of being misunderstood. In the communicative approach, such an error is not a barrier to completing the intended task (particularly seeing as there's a good chance your conversation partner will make the same mistake, given that everyone in your class is a learner), so there is no requirement to attend to it.
Language has a natural tendency to redundancy, in order to make our utterances easier to understand; we are naturally disinclined to attend to every element of the sentence. Therefore any attention to the meaning of all the individual components of an utterance will be a higher-order process, a conscious or semi-conscious overriding of our lower instincts. Surely that makes it an explicit process? And if it is an explicit process, surely it is better for it to be directed by an expert (the teacher) than carried out on an ad hoc basis by a non-expert (the learner)?