Ok. When I "air-squat" that is with no weight I come down smoothly, heels on the floor, chest up, facing forward, spine in a neutral position. All is well.
However when I add weight, I will try and get video this Thursday, my form deteriorates. I lean forward more and I feel more pressure in my lower back. It feels as though in order to get the last few degrees to parallel I actually lean forward and "drop" my butt. It's really difficult to explain.
Now, as for my squatting position. I stand with feet shoulder width apart toes slightly pointed out the bar rests on top of my traps, I squeeze my shoulder blades together as I squat. Arms are tight and my grip is fairly close to help me keep my back tight. I come down smoothly till I reach parallel, then return to standing trying to focus on not bouncing or leaning forward though the latter occasionally occours in my later reps.
Hope this is a little closer to what you needed.