Karmanein Vadhaka Rastey Maa Faaleshu Kadachan-Bhagavad GitaIt means that perform your duty with generosity and forget what will be the outcome of that particular duty.
First part of this tip seems to be clear because you need to keep your mind calm to get better concentration. Now you will ask, "Why should I forget about getting a good rank?". The answer to this question is very simple. Forgetting about a good rank is one of the ways to keep your mind calm. Just assume that you are happy with whatever rank you get. If you do this, you will be able to concentrate more on your preparation than your performance. I am asking just to forget about good rank, not about good preparation. Give your best to your preparation. This way you will be able to remove some of the useless thoughts from your mind and give your best performance in the exam.Usually it happens that even if our preparation is good, we make lots of mistakes in the exam. These mistakes happen either because of anxiety or because of overconfidence. Anxiety comes when you start thinking that what will happen if by chance you don't get a good rank. Overconfidence comes when you start feeling that you will easily get a very good rank. In my view both of these situations are harmful for an ordinary student, because you will not be able to concentrate on the exam. You will be lost in these thoughts. If you assume in advance that you are not going to get a good rank, you will feel neither anxious nor over confident. Hence you will be able to get better concentration in the exam followed by a good rank which will open the doors of IISc/IITs for you.