Brain2 HD switch
You can use any two pins of the 3-pin connector.
To be precise, you need to use the center pin and one of the two "external" pins.
(The highest and lowest pins are connected together. Then one of the two "external" pins should be connected to the center pin)
You can also power the Brain2 HD on one of the servo connectors (EG: CH2). In this case, however, the switch will no longer have any effect because the switch only controls the input power on the MPX connector.
Note: if for redundancy you want to power the Brain2 and/or Brain2 HD and/or Mini Brain2 and/or Micro Brain2 from different connectors, the voltage carried by the two connectors must be identical because all grounding/GND pins are connected together and all +V pins are connected together (on HD this happen for +V of MPX connector when switch is open).