The purpose of the coffee break feature is so that you can leave your computer running, but be secure in the fact that no one can access what you are currently working on without your password.
First, be sure that the security is turned on. Then choose Coffee Break from the File menu. A screen saver will appear with a password entry box in the centre.
When you return to your computer, enter your password and click OK. This will return you to where you left off before your coffee break.