Slack is the team’s/individuals personal buffer time. When the task is complicated and complex like software development are, we need slack time to handle random problems that are impossible to plan ahead. If you maximise and stretch the system too much you risk to break it.
If you don´t buffer money for unplanned expenses you risk to break you finance, its the same in organisations.
Organizations that doesn’t work with slack is either so routine that you can let robots do the work, or the organizations have big trouble in getting the predicted output.
One way to introduce slack is to schedule important, useful work that isn’t time critical. Then you hopefully can work on technical debt, team activities, innovate new products or somethings else that are good for the company if the planed slack time is left after an iteration.
Is tempting to maximize an plan to much and this is often correlated to our obsession to measure 40 hours week. Getting the slack time right is hard so the only way is trying, learning and adapt. If the organizations is really good in estimation the slack time can be smaller, but remember to plan for other task like technical debt in the iteration instead.
Read more at James Shore