This summary is not the rule. The rule text controls.
Lawyers can't agree to limit their own right to practice law in the future just because it benefits them in a job deal or a lawsuit settlement. For example, a law firm cannot make a lawyer sign a contract saying that if they ever leave the firm, they won't be allowed to represent certain clients or compete for business in the same area. The only exception is for retirement agreements, or in situations covered by a separate rule about selling a law practice.
The same restriction applies when settling lawsuits. If a lawyer is negotiating a settlement for a client, they cannot agree to terms that stop the opposing lawyer from taking similar cases in the future. This might come up, for example, if a company wants to settle a case but also wants a promise that the lawyer who sued them won't represent other people with the same kind of claim.
The idea behind this rule is to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing in certain areas, it would become harder for future clients to find good legal help, especially in specialized types of cases. This rule keeps the legal market open so lawyers can keep working freely and clients keep having options.