The number opens up the conversation to:
* Why this metric and not another? How was this number related to the overall goals of your company?
* What was the initial benchmark for how much you wanted this number to change? Did you meet, exceed or fall short of this benchmark? What do you think was the reason for that? In particular, what did you learn about your org's benchmarking process from meeting, exceeding or falling short of the benchmark?
* What resources were required to make this number change? Who were all the people involved? Why was it logical for their involvement to be a part of this number changing? More crucially, were there any people not involved that, in retrospect, you wish could have been involved and why?
* Knowing what you did now, if we were to put you on a similar effort within our company, what do you think you would do differently and how much do you think that would have an impact on that number?
The bullshitters will quickly wilt under the pressure of not knowing enough details to give well thought out answers while the people who drove real impact will get excited to talk it through and lay out both what went well and poorly.
edit: A big part of what I'm looking for with "Impact is King" conversations is the ability to take ownership of impact even in the face of dysfunctional organizations because, surprise, all organizations are dysfunctional!
I don't care about the actual impact you made because that's driven much more by the environment you were placed in than your individual performance. I care that you have a good mental model on how to drive impact and that you can perform at a reasonable baseline where you're significantly above average vs peers at your experience level.