Well, the mechanics of the measure are designed to ensure that an individual doesn't get a double benefit from the measure. If you simply said that the value of your liability is limited to what you realize on disposition, the individual would still have all the accumulated capital losses associated with the depreciation of those securities and would be able to apply those capital losses over any future capital gains.
The way the budget mechanism works is that it attributes a capital gain equal to the losses you've incurred because of the depreciation of the stock, thereby wiping out those losses, and then gives you the forgiveness. I think the clause you're referring to is that mechanism by which we remove the accumulated capital losses associated with the security.