A debit instruction slip (DIS) is your authorization to debit your account with the requisite number of shares. The shares will be directly debited to your demat account after verifying the signature and the balance of shares in your demat account. When you sell shares, there are some basic points to remember about the timelines. Sale of shares will result in pay-in on the next trading day by around 11 am. When we talk about T+1 we are talking about trading days only. So if you sell on Friday, then the T+1 day will be the coming Monday. So the DIS has to reach the broker by T+1 morning. Normally Brokers will insist that the DIS should reach them and the shares should be in their pool account by 9.30 am on T+1 day. This is to ensure that you still have time left with you in case of any objections. If you are signing the DIS for every sale, it is always better for you to submit the DIS to your broker on the same day and get an acknowledgment from the broker. Make it a point to submit the DIS on the day of the sale itself, to be on the safer side.
Is there a simpler way to do it instead of going through all these hassles? You can opt for an internet trading account and link your trading account and demat account. After that, you can execute a power of attorney (POA) agreement with your broker authorizing them to debit your DP account against the sale of shares. If that is done then you do not have to worry about DIS and other formalities. Things will happen automatically in this case.