BTST (Buy Today Sell Tomorrow) are trades where you buy shares and sell them on the next trading day (T+1 day). The risk with BTST trades is that since you are selling shares that aren't in your demat account yet, you rely on the seller you've bought the shares from to deliver the stock to you.
If the seller defaults on delivering your shares, in the event of short delivery, your obligation as a seller to deliver shares won't be met, and you will face the risk of auction penalty, which can be up to 20% of the value of stock short delivered.