It's been asked, but not answered as far as I am aware. I finished the general (non map specific) defend recently, and I now think it has something to do with the "blocked" message, e.g. "Hubert PIckle blocked the Refinery" - which looks like your colour timer has to start to run down when challenged by an opponent, and you push off the challenger(s) and fully reclaim the point.
X captures
Y challenges <- this has to happen to get a block
X blocks <- this is what I think counts as defend
As ever, either Clara or Awkm will have to provide definitive guidance.