pjax loads html from your server into the current page without a full page load. It's ajax with real permalinks, page titles, and a working back button that fully degrades.
Check the box to toggle pjax.
Whenever the time changes, a full page load has happened. If the time doesn't change, no full page load has occurred.
The idea is you can't tell the difference between pjax page loads and normal page loads. On complicated sites, browsing just "feels faster."