Work in progress is your work that has been done up to a particular point but that is not yet billed. Billed-basis accounting versus accrual accounting are two different ways of treating for tax purposes your work in progress. Your work in progress is a thing, and the question is, how is it to be taxed?
The general rules provide for accrual accounting, but this provision in the act allowed designated professionals to elect into a billed-basis method, which would change the tax treatment of how work in progress is taxed. That's how they relate.