Slack is Unplanned Improvement Work

Full utilization cause delays - http://agileconsulting.blogspot.no/2011/07/explaining-why-limiting-wip-is-so.html
Full utilization is not effective, not efficient, and causes delays

Being busy all the time is bad for you, your team, and your organization.  It is far better to allow for slack, which is not a bad thing, but creates opportunities for improvement without needing to schedule them.

“Recipe” for what to work on next, incorporating slack:

  1. Can you help progress existing planned, started work?
    Work on that.
  2. Can’t do that?
    Find bottleneck and help work to release it.
  3. Can’t do that either?
    Do some (improvement) work which

    • won’t create any work downstream,
    • will improve future throughput, and
    • can be paused as soon as existing kanban related work is available.

A bit more reading related to slack.