Problem solving is of critical importance in many disciplines. In medicine, the clinician's ability to arrive at the correct diagnosis often means the difference between life and death. Despite its importance and a significant amount of research regarding how to improve problem solving, few unambiguous answers have emerged for promoting problem solving learning. This paper focuses on explaining those gains, both theoretically, and in terms of the qualitative data collected from learners over the past two years. The goal of the paper is to associate those gains with specific characteristics of the DP (Diagnostic Pathfinder, a software learning tool designed to help clinical pathology students improve their ability to solve diagnostic problems) by and meaningfully categorizing and characterizing thousands of comments from students who used the DP in a number of different settings, and to use those comments to illustrate, from the students' perspective, how using the DP accomplished what it did. These ideas will be tied to current theory regarding the teaching and learning of problem solving.