I've taken it to mean: You don't know it, you might not particularly want to be using it, but you have a specific requirement and a deadline.
eg: "Plumbing in Anger" I would expect to distill enough to fix a sink or install a toilet or something, and tend towards a bundle of task-focused items/outcomes, and less on the "Gestalt of Plumbing".
I've taken it to mean: You don't know it, you might not particularly want to be using it, but you have a specific requirement and a deadline.
eg: "Plumbing in Anger" I would expect to distill enough to fix a sink or install a toilet or something, and tend towards a bundle of task-focused items/outcomes, and less on the "Gestalt of Plumbing".