I'm trying to explain to someone how non-preemptive multitasking operating systems work.
Windows used to be one. It is now, apparently, preemptive, and if you believe that you believe most things.
What it means is that the whole issue is run by the System, and when the System feels like it, it hands over "control" to a specific function to do a small and insiginficant job, and expects control to be returned to it immediately after that task has finished.
A preemptive one differs in that whilst the specific function is still mid-operation, the System can grab control back and go off and do something else.
It's quite simple.
Oddly, this also describes the System which we call the State.
Every now and again, when it's all gone tits-up, it needs re-booting.
It's the only way.