Why is Intraday trading not allowed on contracts in Ban period?
A security is under a ban period for F&O trading if its derivative contracts cross 95% of Market Wide Position Limits (MWPL). This means that the combined open interest of all F&O contracts for all the months taken together for that particular underlying crosses 95% of MWPL.
Open interest is the number of contracts outstanding in futures and options trading on an official exchange at any one time.
A user isn't allowed to open any new positions in securities under a ban period. But he/she can exit already opened positions.