Like Steve-F said, it's for back gear engagement. You pull it out, and now your back gears can be engaged. It enables more torque , and lower speed. You should have some sort of lever to engage the back gear into the bull gear. I have a South Bend, so I am not sure how the Atlas does it.
I do like the indexing pins, which I had them on mine. while they would be low use, they can be real handy... My wood lathe has something similar, and I have used it many times.