Fastpipe is a windows-only optimization that allows the user mode file server to directly copy data into the buffer of applications that are doing unbuffered file system IO. Depending on the file server implementation, this may allow avoiding an extra memcpy. In practice the optimization ends up being mostly academic.
On all platforms, a normal socket/pipe works just fine with PFM.
There is no documentation for usage of fastpipe outside of PFM. You probably could work it out from the headers if you needed to, but for most things it would make more sense to use a normal socket or shared memory.