How do you maintain a conversation when you have no idea what someone is talking about?

Repeat one thing that stood out about the conversation... and add an oh, really, wow, interesting... buys time and - has them think your invested so they further elaborate on the subject...and - fortunately to opt out of it all- you look at the time and mention that you have to go and hopefully you'll catch up on it later

the key is to BS with a good poker face