Mini nifty future has been mostly used for hedging the position of nifty future, as 1 lot of nifty future is of 50 qty and mini nifty future is of 20 qty. Now using the combination of nifty future and mini nifty future, a trader can lock his profit on his position by taking opposite side of trade. I will make it clear using an example: Suppose you bought nifty future at 5700 and now it is trading at 5850 and you feel that upside is still possible, but you need to book some profit (partial profit booking). In such case you can short sell mini nifty future (20 qty), so your net position will now be 50-20 = 30 and now mathematically you are net long 30 qty in nifty future. In other words you have now locked profit for 20 qty, no matter which direction market move. There are many trading strategies which we follow and if you wish to learn them we have special online course for it.