This part of the law is about making sure that when you calculate something called a ‘discontinuance profit’, the result for each person can never be less than zero. It’s part of the rules about changing from one tax system to another and dealing with regular payments.
The law used to say more about this, but it was removed on 1 July 2010. This means that these specific rules no longer apply, but there might be other rules that have replaced them.
If you need to know more about how this works now, you might need to look at other parts of the tax laws that came into effect after 1 July 2010.