When you buy shares, be it in an online or offline account, the shares are automatically transferred to your demat account, you don’t need a POA to do this.
When you sell shares or want to pledge them, that is when the shares are going out of your demat account, this is when the POA is required if it has to be done online.
Traditionally (offline method),
you would have to send the delivery instruction slip asking to debit the shares from your demat and send it to the brokers office. Once the slip is received at the broker, he would debit your demat account.
source:
http://zerodha.com/z-connect/queries...-demat-account