In case of ESOP, in which format do we get the Prequisite gain?

In the case of ESOP, in which format do we get the Prerequisite gain?

I was told to pay the Prerequisite tax after exercising the stock options. I am ready to pay the tax but they are not mentioning when do I get the prequsite gain? Do I get it when I sell the shares after the IPO? Why do I need to pay the tax when I have not received the Prequisite gain??

@Quicko