I don't know much (I'm applying for this cycle), but based on my experience I would say you should definitely know about your research. Every interview I've had, they've asked about the research (and I'm only MD, not MD/PhD), and if you mention you've spent say a year doing research but don't know what you were really doing, it'll look REALLY bad! On the other hand, it's not like they'll ask you specific questions, so I would say prepare a little 1-minute description of what you did (background including the rationale for doing the research, hypothesis, results) that sounds like you know what you're talking about, and feel comfortable asking general questions like "So how exactly do you do that?" or "How would you know if ___ DOES affect ___?". You don't want them to have ANY suspicion that you don't know your research topic like the back of your hand, even if you really don't. :>