People take part in table topic sessions for years, often alongside their education. The sessions equip them with the skills necessary to be able to answer any question thrown at them and to do so with confidence and authority.
Restrict your answers to response templates
Skilled impromptu speech is not a question of coming up with any old answer on the fly. There are actually response templates that you need to learn to wrap your answer in. With practice, these response templates lead to carefully structured answers making you sound much more confident and coherent.