BTST (Buy Today, Sell Tomorrow) is a facility that allows customers to sell shares before they are credited into a Demat account or take the delivery of shares. The decision has to be made in 1 day.
Let's assume you buy 100 shares under BTST today and sell the 100 shares the next day. Now, what if the person who sold you 100 shares didn't deliver to you for some reason. The stock will then go into auction.