Tom DeMarco, co-author of "Peopleware" (1987) with Timothy Lister and author of "Slack" (2001), posed a challenge that the Poppendiecks explicitly engaged with. DeMarco argued that productive knowledge work requires unscheduled time — slack — because creative problem-solving cannot be scheduled. This appeared to conflict with lean's discipline of waste elimination: if all idle time is waste, where does slack fit?
The Poppendiecks resolved this in amplify-learning: if learning is the primary value stream in software, then slack that enables learning is not waste but value-adding time. DeMarco was right that knowledge workers need unstructured time; the lean framework accommodates this once you recognize that the value stream is knowledge, not code. This resolution is one of the Poppendiecks' most intellectually distinctive contributions, making DeMarco a key figure in their intellectual story even though they never collaborated directly.