Hi, there is some info
here
Regarding the hardware design of the programmable one (SW3 which is connected to PWM2), the Arduino Due board has an internal pull-up resistor, so when the pull-up is enabled, it will be in "high" level by default and when you connect it to ground it goes to "low" level.
So the "nothing" you mentioned is the internal pull-up, to configure it
check this topic.