DeMarco's book is not only about Slack--but if you want to read about it see:
"People under time pressure don't think faster"--Tim Lister
DeMarco discusses at length the popular management idea that people under pressure work harder. Sorry--pressure only helps for physical labor. So what are managers supposed to do if they're not turning the screws? They're supposed to protect the focus in the work environment; to build relationships between individuals; to delegate and keep hands out of the "dirty work"; and to fight against aggressive schedules, overtime, a culture of fear, and litigation. Managers are avoid command-and-control behavior, and need to learn to lead by extending trust before it is earned: "you acquire trust by giving trust".
There is no managing of change. No one knows what's going to happen. As Weinberg says, he goes in, shakes things up a little, and waits for concerned employees to come asking for his opinion. DeMarco adds: "in times of change, the reward has to come first". It's hard to change--it can be embarrassing for an expert to learn a new skill and be reduced to the same level as junior colleagues. Anyone who is learning, trying things out, taking risks, is likely to be afraid--but if they're not in a safe environment--they may get ridiculed--and that can kill their motivation to change. Furthermore, change has to be introduced when things are going smoothly! If things aren't going smoothly, and you still need to change--add a lot of slack.
DeMarco says that a focus on process comes from Taylorist thinking. From an observer's perspective, the physical process followed by stellar employees may be the same as the average employee--but that's because knowledge work depends on relationships or abstractions, not physical steps.
It's more important to be effective than efficient; that is, do the right things, before working on doing them the right way. As soon as we streamline our process, it becomes inflexible.
This couldn't be more simply put: "the skills [of management] are inherently difficult to master". Often managers are promoted from technical disciplines without proper training. This produces all kinds of common side effects... for example, the stereotypical weekly status meeting. These meetings may be nothing other than an opportunity for the 'boss' to demonstrate some sort of control over the project. What a manager needs to learn is to extend trust in a qualified context--and to give people the chance to fail. Ultimately this gives people the chance to learn and improve. Instead of second-guessing all the small decisions, the 'boss' can focus on the big picture with risk management. Appropriate risk management means we've planned for many failures along the way to ultimate success.