Forgive me if I don't know all the correct terminology, on my craftsman 12 inch lathe there is a pulley just back from the chuck with a bunch of holes. There is what looks like a pin. What is it for and how do I use it?
It’s for indexing the spindle at set increments, say for scribing the lines on a dial or something. Do not be tempted to use the pin for locking the spindle to remove the chuck — it will fail instantly. I think there are 60 holes, at least there are 60 on my smaller ones.
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.
Pull the pin to release the bull bear from the spindle and engage the back gears via a lever. There is a bit of a trick to get the pin back in if one has not made a mark on the pulley set. Turn the spindle by hand until the pin slips back into the hole properly. Be sure to test before powering up the lathe as damage can occur if not correctly seated.
As pdentrum said pull the pin & engage the lever for using the back-gears. Also, I highly recommend unplugging the lathe while doing this maneuver. Your fingers are in a very vulnerable position either pulling it out or pushing it back in. I've pinched a finger just moving the spindle with my left hand while reseating the pin with my right.