I think you might get better results with a slight change in wording: "What can you tell me about how an index works?" With the wording you used, some intervie

I think you might get better results with a slight change in wording: "What can you tell me about how an index works?" With the wording you used, some interviewees' minds will race to the limits of what they know and then panic as they think about all the things beyond that. A "what can you tell me" question is more likely to prompt them to start with the basics and then expand more comfortably to the limits of their knowledge.