A more thorough explanation:

The inevitable-disclosure doctrine is a legal theory that applies to trade secrets. It states that if a key employee leaves a company and joins a competitor, they cannot avoid using the former employer's trade secrets. This is because the employee has confidential information that they will inevitably use to unfairly compete against their former employer.

To obtain an injunction, the plaintiff must prove that the former employee has confidential information and will not be able to avoid using that knowledge to unfairly compete against the plaintiff. However, most courts have rejected this doctrine because it effectively turns a nondisclosure agreement into a disfavored noncompetition agreement.

For example, in the case of PepsiCo, Inc. v. Redmond, the court upheld the inevitable-disclosure doctrine. PepsiCo found itself in the position of a coach, one of whose players has left, playbook in hand, to join the opposing team before the big game. The court ruled that Redmond, the former employee, had access to confidential information that he would inevitably use to compete against PepsiCo.

Another example is a software engineer who leaves a tech company and joins a competitor. If the engineer has access to the former employer's trade secrets, they may inevitably use that knowledge to develop similar products for the competitor, which would be unfair competition.