A more thorough explanation:

Secondary-line injury is a term used in antitrust law to describe the act of hindering competition among a seller's customers by offering favorable prices to one customer or a select group of customers, to the detriment of others. This practice is prohibited under the price-discrimination provisions of the Robinson-Patman Act.

For example, if a company sells a product to one customer at a lower price than it offers to other customers, it may be engaging in secondary-line injury. This can harm the other customers who are forced to pay higher prices, and it can also harm competition among the seller's customers.

Another example of secondary-line injury is when a company offers exclusive deals or discounts to a select group of customers, which can give them an unfair advantage over other customers and harm competition.

These examples illustrate how secondary-line injury can harm competition and lead to unfair pricing practices. By offering favorable prices to some customers and not others, companies can create an uneven playing field and harm the overall market.