You will get an opportunity to do a Practice Interview, these can be taken several times so you can get comfortable with the process. It also gives you a chance to ensure that your webcam & microphone are working. When you log in and click on 'get started' you'll see the button to "Start Practicing" after the testing phase which brings you into this stage of the process.
When you've completed that and are happy all is in order. You will proceed finally to start your actual interview. This is your real interview and you can only do this once. As soon as you click on "start interview" you are committed to this stage of the process.
Depending on the client you are interviewing for you may or may not see the option to "review your interview"