From CM Volume 1, Preface:
This adjunct of the will is familiar to us as diversion, whose office is to ease us for a time from will effort, that we may ‘will’ again with added power. The use of suggestion–even self suggestion–as an aid to the will, is to be deprecated, as tending to stultify and stereotype character. It would seem that spontaneity is a condition of development, and that human nature needs the discipline of failure as well as of success.
Diversion, giving ourselves something else to think about for a little while, is ok. Suggestion, which according to Wikipedia means to "guide the thoughts, feelings or behaviour", either of oneself or of someone else, is not ok. Trying to manipulate the child, or get the child to manipulate himself, out of the undesired behavior into the desired behavior is not recommended because it does nothing to train and strengthen the will. Instead, we must work with diversion, which requires us to be creative in coming up with diversions, and take our chances that sometimes we will fail. Failure may be necessary as part of the learning process.
Pages 82-83 of Volume 6 (the following parts of the passage, beyond this quote, also deal with suggestion):
’Suggestion’ goes to work more subtly. The teacher has mastered the gamut of motives which play upon human nature and every suggestion is aimed at one or other of these. He may not use the nursery suggestions of lollipops or bogies but he does in reality employ these if expressed in more spiritual values, suggestions subtly applied to the idiosyncrasies of a given child. ‘Suggestion’ is too subtle to be illustrated with advantage: Dr. Stephen Paget holds that it should be used only as a surgeon uses an anesthetic; but it is an instrument easy to handle, and unconsidered suggestion plays on a child’s mind as the winds on a weathercock.
Pages 129-130 of Volume 6 (there is more about suggestion in this section than what I am quoting):
From the cradle to the grave suggestions crowd upon us, and such suggestions become part of our education because we must choose between them. But a suggestion given by intent and supported by an outside personality has an added strength which few are able to resist, just because the choice has been made by another and not by ourselves, and our tendency is to accept this vicarious choice and follow the path of least resistance. No doubt much of this vicarious choosing is done for our good, whether for our health of body or amenableness of mind; but those who propose suggestion as a means of education do not consider that with every such attempt upon a child they weaken that which should make a man of him, his own power of choice.
When you enforce a natural consequence, that allows him a choice. I can be quiet and hear a story, or I can choose to make noise and miss the story. "Suggesting" a better course of behavior is not "suggestion" in this sense. It’s ok to suggest things. It’s not ok to use the specific tool of suggestion to try to manipulate a child. Stumped for a better explanation, I just called my dad, who has a degree in psychology. After talking with him, here’s my best attempt at a definition:
Suggestion is using irrational fears or hopes to coerce a desired behavior.
If you don’t eat your broccoli, you’ll grow up to be sickly.
If you eat your spinach, you’ll grow up to be strong like Popeye.
Those are silly, but they hopefully convey a bit of the sense of it. The child isn’t eating the food because he should but because he is afraid of some bogeyman or hopeful for some unnatural reward (being strong like Popeye is not a natural consequence of eating spinach nor is being sickly a natural consequence of not eating broccoli). Natural consequences allow the will to become stronger, and they respect the personality of the child and his right to choose, even if his choice carries with it negative consequences.
I’m not entirely satisfied with that explanation, though.