Presto – Normally when you update a package in Fedora, you download an entire replacement package. Most of the time (especially for the larger packages), most of the actual data in the updated package is the same as the original package, but you still end up downloading the full package. Presto allows you to download the difference (called the delta) between the package you have installed and the one you want to update to. This can reduce the download size of updates by 60% – 80%. It is not enabled by default for this release. To make use of this feature you must install the yum-presto plugin with yum install yum-presto.
http://docs.fedoraproject.org/release-notes/f11/en-US/
https://fedoraproject.org/wiki/Features/Presto