How does BIND 9 use memory to store DNS zones?
When reloading a zone, named may have multiple copies of the zone in memory at one time: the zone it is serving and the one it is loading. If reloads are ultra-fast it can have more still, e.g. ones that are transferring out, the one that it is serving, and the one that is loading.
BIND 8 destroyed the zone before loading and also killed off outgoing transfers of the zone.
The new strategy allows secondaries to get copies of the new zone regardless of how often the primary is loaded compared to the transfer time. The secondary might skip some intermediate versions, but the transfers will complete and it will keep reasonably in sync with the primary.
The new strategy also allows the primary to recover from syntax and other errors in the primary file as it still has an in-core copy of the old contents.