The commentary was happening through Mumble server that SRC put up. For game a/v, I believe Festy used OBS to just direct it to whatever stream setup SRC was using. There was about 3-5 seconds of delay for me watching the stream (Festy of course was watching it real time), so sometimes the commentary was de-synced, or I reacted late.
Normally when I watch/comment, it's either dealing with severe twitch delay, or if it's one-on-one/simple, just screenshare through Skype or hangouts or what have you.