[davnicwil]

I think the trick here is have people talk through what it does, how and why (as in what are the tradeoffs, what other approaches could have been taken, etc).

You can't fake this. Or to put it better, even if the code isn't yours and you can do this well, it doesn't even matter that the code isn't yours as in doing this you by definition have the skills and knowledge to reimplement it anyway.

[pmikesell]

You can absolutely fake this. There’s a huge difference between coming up with and implementing something vs understanding it enough to be convincing.

Have you actually tested this out?

[davnicwil]

Yes I've done this both as an interviewer and interviewee, it's a pretty valuable exercise I've found. Not a silver bullet, not the only thing you can do, not appropriate for every situation, but generally good.

Regarding your other point I agree with you, but if the main point is to evaluate understanding then the difference doesn't really matter. If you're testing for ability to invent concepts etc, then this might be more important to you.

[dcolkitt]

Okay, but at this point it sounds pretty much the same as a traditional interview where the candidate discusses their prior experience.