Easiest is like netbanking for your bank account. You can credit and debit shares by yourself without giving the power of attorney to your broker. This prevents unauthorized transactions on demat.
As of now the common practice is to sign a Power of Attorney giving the broker the right to modify the balance on demat account without you doing anything. Its like giving the power of attorney to the bank manager to debit your bank account in a sense. The advantage is that a new trader is insulated from the mechanics of settlement and the broker is not exposed to non delivery of positions that are not there on the demat. The downside is that if the broker does fraud by selling client stocks because they have a power of attorney, your claim will be attached to the overall claims against all people and your stocks which should ideally be not affected by broker default, will be part of it.