It uses unix sockets. Latency is the same as the standard pre-forking model; the only difference is that file-descriptor passing lets you manage the worker processes independently instead of requiring them to have a common parent process (this is important when rolling out new code to a service with a lot of active connections, since it's disruptive to restart all the workers at once).

Here's a demo in Python: https://gist.github.com/bdarnell/1073945