The key to making profit in this strategy is :
When you enter this trade say on day 1....on day 2 you need to exit whenever you get comfortable profits.....if the trend is up and you have bought banknifty and sold nifty on day 1, your net position will be in profit and you can hold on to the position....in this case if trend turns negative on day 2 and banknifty ltp goes below pp convincingly with good volumes, it would be advisable to exit both the positions with gain....
If the trend is in your favour throughout day 2, you can just watch your profit and keep monitoring the banknifty ltp vis-a-vis pp at every time....if at 3.27pm on day 2 also the banknifty ltp is higher than pp, you may carry forward the positions to day 3 or if your finances permit, you may buy & add 1 more lot of banknifty and sell 2 more lots of nifty.....
Again repeat the above procedure on day 3....remember to monitor banknifty ltp vis-a-vis pp at any given point of time...