The Test drive button allows you to test the portal as one of your customers. This allows you to see the portal from your customer's perspective.
When clicked, the right panel opens up showing recently viewed contacts. You can also search for an existing contact.
By selecting a customer, you will view their portal. At the top is an indicator reminding you who you are logged in as.
Note: Any changes made and saved while signed in as that user are saved for that user! Take care when making edits.