This paper describes the results of a study investigating the process by which writers develop their understanding through writing. It argues that, contrary to problem-solving models of writing, the crucial ingredient is implicitly guided text production. Two groups of writers, varying in the extent to which their writing is assumed to be directed towards rhetorical goals, were asked to write either planned or non-planned texts. Key-stroke logs were collected, and changes in subjective understanding about the topic were measured. The results show that developments of understanding are strongly related to the extent to which writers modify their texts during writing, and this is highest in the conditions expected to promote implicitly guided text production. We conclude that these findings support a dual-process model of writing.