I (of course) think of it like multi-tasking in a computer. The machine needs to capture the context (stack and registers) associated with one task in order to switch to another. However, for a human this context can be enormous. Depending on the task, it is possible that one will never get back to the state of understanding that they had at an earlier point if working on a particularly complex thing - just the right combination of caffeine, sleep, motivation (more accurately gumption if you have read ZAMM), etc.

The ZAMM reference is spot on to me. The moment I used to get an interruption (typically an incoming call, but not only that), what happened to me was just what Pirsig describe of the gumption leaving you like air from a baloon, psssst.

I eventually got into 'the zone' and I was actually quite good at figuring out problems with little context, but my gumption was gone nevertheless, and I was useless for whatever thing I was working on before the interruption for mostly the rest of the day.

Yes, I think it is the most important software engineering book by far.