Oh, by the way, excellent question! I'd like to know that answer as well
Hey Sergey -
When a refresh occurs, the "original" extract continues to be leveraged by "current" users while the "new" one is prepared. Once the "new" one is ready, any "new" users (they create new session) who ask for data use the "new" extract. After the last of the "current" users stop using the "original" data source, it can be destroyed by the extract reaping process which runs on a schedule in Server.
Russell, thank you for the fast response!