The narration interrupts the order of time and returns to events earlier than the point the story has reached.
In The Magic Mountain
So the boy approved in his heart that the grandfather should appear in full authenticity and perfection on the day when the hour came to bid him farewell forever [...] the grandfather, solemnly released from that passing adaptation, had definitively assumed his genuine and deserved appearance
Chapter II interrupts Hans's arrival at the Berghof and returns to his childhood to recount the grandfather's wake. The remembered scene shows where the boy's familiarity with death comes from, before the story returns to the mountain.
Chapter II: The Baptismal Basin and the Two Figures of the Grandfather, the narrator