According to regulations (PDF), the trading window for a particular security will be closed for designated persons of the company who are expected to have unpublished price-sensitive information (UPSI). A "no trading window period" is placed on such securities for designated persons.
During the "no trading window period", no trading of any kind will be allowed, including exiting existing positions. You need to exit all your open positions before the trading window closes. The company will communicate the duration of this period to you as a designated person and will also specify it on the company's website. However, you can use Immediate or Cancel (IOC) order type to trade in any other instrument except for the blocked security.
You can submit a declaration to [email protected] for exemption from the closure of the trading window. You can find the declaration in the CDSL circular (PDF) on designated person trading.