The linked chapter 5 of the legendary unitrode Magnetics Design for Switching Power Supplies seminar paper focusses
on a visual understanding of the gapped core's operation. For a fundamental answer on the raised question, it's
meaningful to my opinion, to reflect also the basic laws that rules a magnetic circuit. They are mainly discussed in the
seminar papers first section Introduction and basic magnetics. https://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014