It's like streaming the game but the viewer can join the game as 2nd player. So if games have splitscreen/ local coop support, then it's possible to play together even if the viewer doesn't own the game. The speed requirement is quit beefy though. It's 10 Mega bits/s of upload and 2 Mega bits/s of download speed for the host and 10 Mb/s download and 2 Mb/s upload speed for the client [viewer].
Games like Fifa, Pes, House of Dead worked just fine. Old NFS games that had splitscreen also worked. If anyone's interested in playing these games together, you can send me pm here.
Some gameplay footage of playing via parsec:
Resident Evil: The Darkside Chronicles
Need For Speed II
Edit: Due to lack of interest, I'm retiring this thread.