As per my limited knowledge, write off is done against balance sheet.
Provisioning is done from Profit and loss statement. So there is a reserve of provision which gets money from pre provisioning operating profit .
Then later when required Non performing loans are written off at the expense of that reserve/provision. If the loans are written off and there is not enough provision then it will eat into bank’s equity. That will reduce share price since banks are valued in terms of PB ratio and the book value is nothing but the equity.
Suppose I have 30 rs (this is my equity) and I borrowed 70 rs at 0% interest to give a loan of 100 rs at 7% rate of interest. So I earn 7 rs at year end. As a prudent bank I have set aside 3 rs as provision from that 7 rs. So my net profit becomes 4 rs and after paying tax lets say I have left 3 rs which gets added to my equity. Now my equity becomes 33 Rs. Now next year also I give a loan of 103 rs but this time out of that 103 Rs lets say 1 Rs. becomes NPA and I can’t recover the money. Now since I’ve a provision of 4 rs so I can write off that 1 rs loan from my provision. Now lets say I have to write off 4 rs NPA. Then I have 3 rs provision and 1 more rs will be taken out of my equity of 33 rs and this will lead to provision becoming 0 and a loss of 1 rs.