A JMockit developer uses the JMockit toolkit for software unit testing in Java. They create mock objects, fake implementations of real system components, to isolate and test individual units of code. They design test cases to simulate scenarios, manipulate behaviors and responses of dependencies, and validate outcomes. They also use code coverage tools in JMockit to ensure all code paths are tested. Additionally, they may use JMockit's APIs for mocking, faking, and integration testing. Their work is crucial for verifying code functionality, ensuring software quality and improving the reliability of the application.