This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
Transaction demarcation is used when you want the application server to commit the transaction for you. It is most often used with EJBs where you define the transaction needs of the methods and the app server automatically determines when to commit the transaction.
If, instead, you want to control your own transactions, then call commit on the Transaction Manager within your code.