This is specific to Amazon Web Services: if I understand correctly, this essentially consists on passing the bulk of the data not directly form a server in one Availability Zone (data center?) to another, but to store it in S3 and pass only messages about it (metadata) which are tiny in comparison.
Apparently, transmission between any AZ and S3 is flat rate while AZ<->AZ is metered which is where the savings come from.
Apparently, transmission between any AZ and S3 is flat rate while AZ<->AZ is metered which is where the savings come from.