Presumably those circumstances would depend upon the impulse in question. I do not think Russel means to imply that all important work springs from a single, common impulse. I think the impulse indicated is unique to the work, and that in the case of each work, circumstances must be found that allow the work's evolution.
I think he is talking about the adult version of something like Montessori's concept of 'horme'. A work has an internal energy, which it naturally generates more of as it succeeds, so what is most important is not to block or interrupt its natural evolution. Our reflex is to help children, to speed up the process with guidance, or rewards. But that is just our excessive empathic investment, and our impatience. What we really need to do is allow the natural maturation to happen on its own and ensure the environment contains enough interesting variation that the skill will find motivation to display itself.
The things that we all need to learn, like arithmetic, or reading, have a sort of common natural evolution. You cannot teach them all the same way. But we can discern what is needed for each, and guess how to enable each child in each realm, without pulling them off course.
The important intellectual work of adulthood is more unique. We are not converging toward a known point where others have gone before us. We are developing original directions. So beyond the obvious, what is needed most is attention to "affording scope".