Questioning involves inquiry to construct models. Questioning to learn challenges, confirms or refutes hypotheses.
A context where I used this skill:
I recently used "questions to learn" as part of task analysis. I interviewed end users to learn how they did their job. I built storyboard models of the usage scenarios. I asked users open ended questions to share their recent work experiences. During the interviews I asked clarifying questions to confirm definitions and ensure I had a common understanding with the interviewee. I reiterated the story in chronological order so that I could identify missing steps, gaps or incorrect ordering of activities. I asked questions about any objects identified. For example how was the object created, how was the object changed, who was responsible for the object. I ask questions about any actions identified in the story. I looked for omissions. I explored alternatives with some what it questions anchored on decision points and choices identified by the end users.
How I'd recommend someone learn this skill:
I suggest the first step is for testers to observe some interviews with end users for the purpose of describing workflows or usage scenarios. Review some articles about the subject, for example The Key to Good Interviewing by Robert Sabourin.
Literature on the topic of problem solving also offers insights into questioning to learn. How to Solve It: A New Aspect of Mathematical Method by G. Polya
I suggest that testers listen to recordings of interviews and critique them with experienced practitioners and perform interviews and then have the results reviewed with experienced practitioners and peers.