You can do BTST (Buy Today Sell Tomorrow) trades at Zerodha by buying the stock using Longterm (CNC) product type today and selling the same stock tomorrow using Longterm (CNC).
After you buy stocks and they are credited to your demat account, Zerodha earmarks these shares, and you sell them on the next trading day. For example, buying shares on Monday and selling them on Tuesday is considered a BTST trade.
Even though you do not directly sell shares from your demat account, DP charges are still applicable for BTST trades.
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.