Showing the nested sub-navigation of the active primary link

This seems obvious, yet every time I start a new Drupal project I spend at least 10 minutes looking at older projects trying to find the solution.

I'm talking about getting Drupal to show the sub-navigation of a primary link when that primary link is active.

The solution is so simple it invariably leads to a facepalm: Instead of using the provided $primary_links variable in your template, enable the primary links block on the block configuration page.

Why can't I remember this?