An interesting (and true) paragraph from Slack (page 17):

There are certain kinds of work that require immersion into the task before progress can begin. In this category I include jobs like writing, research, analysis, invention and programming. The immersion time is required to overcome a kind of mental inertia. Most of us are reluctant to begin on such a task unless we have a large block of time set aside.

If you know or manages these kinds of people, please repeat them from dusk till dawn until you can recite the paragraph letter-by-letter (emphasis mine):

The immersion time is REQUIRED to overcome a kind of mental inertia. Most of us are reluctant to begin on such a task unless we have a LARGE block of UNINTERRUPTED time set aside.

I beg you, please do. (I'm kidding, of course :)

But this is a 2002 book. Yet, here we are in 2010 and there're still truckloads of managers who still doesn't have the tiniest clue.

Wait!.. There's more (picture me springing up from my bed just to include this):

When highly immersive tasks are interrupted, a second immersion period is required to get going again. There may also be a frustration component if interruption is frequent. Workers who are frustrated have to use up some of their energy and time to calm themselves and face up to the potential for further frustration when they begin again.

Programmers will talk to you about being "In the zone" and this is what they meant. You'll get The Snap if you interrupt them, which is what this guy meant by frustration.