quote is from Highsmith's Plotting and Writing Suspense Fiction

I love the concept of "slender ideas" as small, fleeting notes which might accumulate into something if written down. In saying "Close the notebook and think about it for a few days" Patricia Highsmith seems to be suggesting that one engage in diffuse thinking, passive digesting, or mulling rather than active or proactive thinking.

She also invokes the magic word "presto!" (which she exclaims) as if to indicate that magically the difficult work of writing is somehow no longer difficult. Many writers seem to indicate that this is a phenomenon, but never seem to put their finger on the mechanism of why it happens. Some seems to stem from the passive digestion over days with diffuse thinking, with portions may also stem from not starting from a blank page and having some material to work against instead of a vacuum.

From Patricia Highsmith: Her Diaries and Notebooks: 1941-1995 (Swiss Literary Archives)