I think Chris is talking specifically about teaching.
It's only partly about finding positive things to say, it's also partly about telling the student what they did right in contrast to what they did wrong. When I am grading a math problem, it's pretty unambiguous that the answer is wrong. So, we start from there: NO. WRONG. After that, though, I take some time to go through whatever work the student did and try to sort out the good ideas from the bad, and point out each. My remarks usually go something like 'good start, you've set this up backwards though, you should have
started out this those two things the other way around. Then at this step you lost the minus sign. And here you seem to have just gone crazy so everything after this makes no
I direct the student toward more of This, and less of That.
It doesn't mean that the picture is salvageable, it may well still be NO. WRONG. But there's almost always something good in a picture that's worth pointing out.