Railgun opens a secure, tunneled connection between the CloudFlare network and Jub Jub's servers where the connection only sends differences from the last request. This is similar to how video encoding works.
The markup of websites does not change that frequently from one request to the next. Instead of transferring the entire request between CloudFlare and the origin server, Railgun will transfer only the changes in markup from one request to the next. This cuts down on bandwidth, transfer time, and overall page load times. Railgun caches these differences in memory to make page processing as fast as possible.