This summary is not the rule. The rule text controls.
Lawyers cannot agree to limit their right to practice law in the future as a way to settle a business or legal dispute. This means law firms cannot make partners, associates, or employees sign agreements that stop them from practicing law after they leave the firm. The only exception is an agreement about retirement benefits, since those deals often naturally include some limits on continuing to practice.
The same rule applies when lawyers settle a case for a client. A lawyer cannot agree, as part of a settlement, to stop representing other clients in similar cases against the other side. For example, a lawyer suing a company cannot accept a settlement deal that requires them to promise never to sue that company again on behalf of anyone else.
This rule exists to protect people's ability to choose their own lawyer. If lawyers could be bought out of practicing certain kinds of cases, it would become harder for future clients to find good legal help. It also stops law firms from using these restrictions to control their employees or partners after they leave.