When an employee is approving their performance review, they have the option to send a notification to their manager. This notification uses the Performance - Notify manager, review requires approval. communication template found in Settings > Other > Communication Templates.
Unfortunately no similar functionality exists to allow a manager to notify the employee.
The system appears to be designed with the idea that the employee would always approve the performance review first, optionally notifying their manager, and upon their manager's approval, the performance review would move to the next step in the performance review process, which would also send a notification. That is, it does not account for a situation where a manager would approve the performance review first and then want to prompt the employee.