Absolutely no doubt that when you are asked to speak on some unheard unanticipated topic during MBA selection process, you need to hold your nerves before you start your extempore speech.
Above is possible with strong practice and reading habits on variety of subjects. Aspirants need to be conscious of fact that extempore speech is a test to judge your communication, expression of thoughts, confidence, gestures and art to hold your audience till you conclude with some take away message.
The more you maintain eye contact , it is better as not only you will gain further confidence but you will be able to maneuver your thoughts and change stance during your extempore speech but you must not get out of focus from topic.
It is advised that listening to discussions n AIR features on specialized topics will help you to gain confidence and knowledge and this will get you success in extempore speech.