The essence of cutting AWS data transfer costs by 99% is to use Amazon S3 as an intermediary for data transfers between EC2 instances in different Availability Zones (AZs). Instead of direct transfers, which incur significant costs, you upload the data to S3 (free upload), and then download it within the same region (free download). By keeping the data in S3 only temporarily, you minimize storage costs, drastically reducing overall transfer expenses.