Home of the USB2AX
This is advanced stuff!
On the bottom of the PCB of the USB2AX are exposed eight 0.1“ pads. They provide access to some useful pins of the ATmega and to ground.
To get to them, you have to remove the clear piece of heat-shrink tubing that protects the board. Be careful to protect the board again before using it!
The solder jumper (closed by default) links TX and RX together to form the DATA line. This is used to achieve the half-duplex behavior needed to talk to Dynamixel servos. By cutting the trace between the two solder pads, you can untie them and use the USB2AX as a normal USB to full-duplex TTL serial. Then, if you want to get back to the original behavior, just solder back the two pads together.
Notice that ground is common between the USB plug, the MCU and the Dynamixel connector. The middle pin of the Dynamixel, labeled 12V, is not connected to the rest of the USB2AX circuitry, but it is connected to the power line of the servos, never short it with any other pin!
The three pads on the side of the board are useful to reset the board and to run the stock USB bootloader. This is very useful when developing a custom firmware: if you break the auto-update feature (or simply don't have one), you can always run the bootloader and upload another firmware.
It is recommended not to try to use these as GPIO unless you really know what you are doing.
The pads under the Dynamixel connector give you 4 GPIOs or access to the ATmega's SPI hardware.
The ICSP programming interface uses the SPI pins but might also need access to VCC. In this case, carefully get VBUS on the USB connector (see schematics).
See the ATmega32u2 Datasheet for more information on these signals!