When you buy shares on stock exchanges, it usually takes 2 days (T+2) to settle the same, which means it will take 2 days for the shares to be delivered to your Demat account.

BTST option on Fisdom is an opportunity for you to exit the stock before it is delivered to your Demat account. The same needs to be done on T+1, i.e. one day after you have purchased the shares on the stock exchanges.