What switch? XD the arduino Works with 5v signal levels wich would kill the Sd and the resistor voltage dropper does the Job unsafely. Thats why the logic level shifter is mandatory. You put 3.3v to the ref pin (vcc) then all signals on the A pins are converted down to a stable 3.3v and a stable current onthe B pins
have a look at the middle picture in http://gbatemp.net/threads/3ds-unbricking-progress.361171/page-16#post-4904962 . i just wanted to make sure if that swirtch toggled input or output voltage, as i belive it's used to set the input (so the shield can also be used with 3.3V logic microcontrollers by skipping the conversion)