The D2000's JTAG pins can be muxed with other functions (including UART1, PWMs & GPIO's). JTAG pins are muxed out by default at POR and once they are assigned to a different function, it is impossible to access the TAP without a reset.
e.g. Because UART_B signals are multiplexed with the JTAG signals, by using UART_B, your application disables the JTAG interface.
It is essential that the JTAG TAP be accessed before any pin muxing happens and a function is provided to implement a BootROM hook to guarantee enough time for a JTAG access.
The "pull pin 0 low" feature it stops the application from executing thus allowing JTAG to be used.
By grounding Pin 0 on the CRB you can access the JTAG Interface (to allow a new ROM / Application to be flashed). Jumpers J7, J9, J10, J12, and J13 should also be set to their default positions.. shown below.