I'm guessing the grandparent post is talking about working on multiple (2, maybe 3) tasks. While waiting for something from one task like a compile/test run to complete or a response from another developer on a question, you can work on the other. This allows you to fill empty time with something productive and lets you context switch at favorable times.

That kind of soft context switch is fairly easy to manage. A task coming and forcing a context switch in the middle of something incurs a much higher penalty.

Still sounds like plain 'ol multitasking to me, as does the other reply.

But with multitasking you're working on something at the direct cost of working on something else and indirect cost of time lost switching context. With this, you work on things while other items are being autoworked on (e.g. running a test or a crawl, downloading a file).

Not exactly. As kylec said above it's a different kind of multitasking. But, in a pretty real way there is no way around this kind of multitasking. If you need the results of some long running task you don't stop existing. As a result you are doing something while waiting for those results, even if that 'something' is just staring at the wall.

This is multitasking in the sense that over a period of time you will have worked on multiple things. But it isn't multitasking in the sense that you aren't doing two things at the same instant.

If I understand it correctly, interleaving seems to be like cooperative multitasking, while being interrupted by the manager seems to be like preemptive multitasking.