In common usage or if you have been coming from years and years of management, it probably has negative connotation, as “he/she has been slacking off”. As a manager, he is considered successful only if everyone in the team is fully loaded with tasks and he has completely utilized his team.

Whereas in the agile methodologies and Practices recommend slacking off. Is it important? if so How does it help? What are the challenges in implementing it? Let us look at analogy to understand the importance of slack. If I have to start writing an article, we just start with a blank paper. Just because it has to be fully loaded or full, I fill it with words and words with words without spaces. Will anyone be able to comprehend its meaning? Not really. It needs white spaces between words to make the article meaningful. Slack is like that white spaces. For a lofty physical achievement we achieve by using a lot of muscle power or brute force. For the work that involves thinking or using the mind to achieve more we need to do the opposite of what is done in the physical world. We need to relax and slacken. IT world is Knowledge industry. Knowledge industry uses cognitive skills which need slack to be built into our planning. Having a small coffee break , playing for 10 mins or discussions with the colleagues could energize, recharge an individual and make them productive.

Whereas at a team level implementing some of the agile practices such as planning and innovation iteration or long term solutioning such as emergent design or continuous refactoring creates slack and in turn, will help the team to be more productive.

Here’s how it works : throughout the sprint your team follows the practices of emergent design and refactoring. Then see what your resulting (optimal) velocity is. Use this velocity in future sprints, so that time for good design and refactoring is built in. It may be non-intuitive at first, but these practices will help you go faster in the long run. But it is not easy to implement as most of the times, teams are under pressure from clients or senior management to meet deadlines or commitments set.
To work come those challenges
1. Work with executives to set the thought process. A bunch of buggy features in the short term where customer does not get any value or a couple of valuable well built features? You make a choice.
2. Share some data on benefits of Implementing these practices from the other organization and get buy in.
3. Provide the team with the environment so that they can set their goals and fail early.

To read more about slack Ref : Slack by Tom DeMarco.