A JTAG developer uses the Joint Test Action Group (JTAG) standard to debug and test integrated circuits. They design and implement tests to identify faults in circuitry, ensuring optimal performance and reliability of hardware. They may also use JTAG for programming devices. The developer interfaces with the JTAG port, using specific commands to manipulate the system's operations. This includes accessing system memory, altering register values, observing system state, and even injecting faults for testing purposes. They often work closely with hardware engineers, helping to diagnose and troubleshoot system issues, thereby playing a critical role in product development and quality assurance.