PokerTV is a feature that allows you to observe a live tournament that's in progress, on a 3-hand delay. To do this, simply click the "TV" icon next to a tournament in the lobby with an "in-progress" status (scroll to the bottom of the lobby list to find these games that have started):
Then, enter the screen name of the player you want to watch. Just start typing part of their name and a list of matching names will appear. If the tourney includes a bounty then you'll also have the option to follow the bounty instead of typing in a player's screen name:
Click "Observe", and when the buffering period expires you'll be at their table and able to chat with other observers!