How do you incorporate discretionary behavior into a job description? Prescribed duties are easy, but what about the discretionary part?

When I was 17, I dropped out of high school and worked as a dishwasher at a restaurant. I quickly learned something about systems-thinking that stuck. In the middle of the work station sat a huge dishwashing machine. Temperature gauges, auto soap dispensing and a 90 second cycle timer. Whenever I placed a rack of dishes into the machine, there was a minimum 90 second cycle. No matter what I did, I could never go faster than the machine. The machine, in manufacturing terms, was my bottleneck. Herbie. My mantra was to keep Herbie working. Except for a few seconds each cycle, to move one tray out and a new tray in, my focus was to keep Herbie in cycle.

That 90 second period was my discretionary time. I could soak silverware, rack glasses, stack plates. It was my discretionary time that determined my throughput. If I kept trays in the queue, I was most effective. Whenever Herbie sat idle, I was losing ground. It is the discretionary behavior that determines effectiveness. How does a Manager capture that from team members?