This one is for reference by my players, since at least one has access to Striking ST (Surprise Attacks Only).
Short version - it only works when the target is both surprised and unable to defend. If either is not true, sorry, it has no effect. That's why it has a (steep) discount of -60%.
The whole thread is here (and dates back originally to 2009!)
The (most relevant) post is this one by Kromm.