The examiner has the right to stop you anywhere in between the exam. They must take care of the timings during the paper. Part 1 is strictly not for more than 5 minutes. Hence, they might stop you in between your answers to cover up the time. Because they have lots of candidates per day for the exam. As well as that they might also ask you to speak more to get more ideas from you on the topic. Or sometimes if you do not elaborate much, they prompt questions for you to answer more.