When there are no runnable idle threads.

It's confusing, but "idle" is a priority class as well as a state: https://docs.microsoft.com/en-us/windows/desktop/procthread/...

The idle task (when there's nothing to run) doesn't have a priority at all, it just gets run when nothing else is available. the "idle" priority is an actual priority, for when there's no higher-priority task to run. The ZPT is prioritized below "idle", so super-duper-low priority.

It runs when the there are no higher-priority processes to schedule on a particular core. IDLE processes are higher priority, so as long as an IDLE process can be scheduled, the zero page process won't run.