Earlier quoted context omitted.
This is good advice but with a caveat: some of the muscle control required for particular signs is not able to be learned by children until they're a bit older. For example, voluntary supination/pronation of the forearm is generally not something a 9month old can do. If you try and teach them a common sign for "enough/finished" (fist closed, thumb pointed out, then rotation of the forearm back and forth), or "done" a…
Yeah my daughter used to stick out her index finger and wave it back and forth for finished. One of my favourite memories of that was when we went to see the Vivid light show in Sydney and there was a contortionist on the street so we stopped to watch. I looked into the stroller and said "What do you think?" and she made the sign for "finished". So we moved on.
This is interesting. My son also did "index finger up" in response to "thumbs up" for the longest time. Why is the thumb so hard to manipulate? Late addition to the evolutionary sequence?