Polaroid one step land camera black knob: What does it do?

[unicornsarecoolkthx]

Ok, I have a polaroid land camera, there is a black knob you turn with white marks on it with an arrow pointing above the knob. Okay, There is a little window thing beside of the lens that you look through, The knob is right under it, I found out it is not the flash, so what does it do?

http://farm3.static.flickr.com/2227/2352655998_b87fee96e6.jpg?v=0

Picture ^^^^^^