Pre opening session in Indian stock market is implemented from October 2010. It is basically a session in which people can place the order in few selected stocks and in NIFTY and SENSEX lot, but actual trading will not takes place in this session. Its timing is 9:00 AM to 9:15 AM of trading day.
The value of stock, Nifty or SENSEX in this Preopening Session is only indication of best value of stocks to sell or purchase.
If you see large variation in price of stock in this session, it is because of someone put order to sell or purchase of stock at much varied price with respect to closing price of stock of previous day in very large quantity. They know that it is only a game, actual trade will not take place. They cancel the order soon after placing it.
In pre opening session, your offline trade order will not get executed. There is no worth of preopening session stock price. But, just before actual trading session starts price of stock in preopening session is almost equal to the opening price of stock in actual trading session i.e. at 9:15 AM.