When planning and making project commitments, it is important to  include some slack –  useful but not time-critical tasks that can be dropped if the team is falling behind.

Slack helps the team consistently meet commitments and builds trust and credibility. This re-inforces XP’s value of open and honest communication.

Over commitment leads to poor morale and strained relationships, including rushed, poor quality code.

In “Slack: Getting Past Burnout, Busywork, and the Myth of Total Efficiency.”, Tom DeMarco argues that slack is important for innovation. If team members are packed full with urgent time critical tasks, they wouldn’t have time to experiment and innovate, or see opportunities for innovation .