A more thorough explanation:

Before-and-after theory is a method used in antitrust cases to determine damages for lost profits or overcharges. It involves examining the plaintiff's profits before, during, and after the violation to estimate the reduction in profits caused by the defendant's violation.

For example, let's say a company was found guilty of price-fixing and caused a competitor to lose profits. The before-and-after theory would look at the competitor's profits before the violation, during the violation, and after the violation to estimate the reduction in profits caused by the price-fixing.

The theory is often used in conjunction with other methods, such as the yardstick theory or market-share theory, to determine damages in antitrust cases.