The moment you are told to speak on the topic which you were unfamiliar with that; few moments before, you are bound to get nervous and might fumble while speaking. Exactly here is the test to judge your confidence.
You need to hold your nerves and synchronize your thoughts at the speed which must match your speaking. Your prior reading habits, your preparation and practice will be very handy to match the timings of the occasion.
You must take a pause before you start and during these moments decide to focus on topic. After that you must not stop till your assigned time is getting over. Don’t go out of focus of the topic and don’t ever forget to conclude in positive way. Your communication power, language and attitude will supplement the confidence level to be successful at Extempore Speech.