The bar travels on a fixed track, so it can only move straight up and down rather than through the slight arc a free bar takes. Set the bar across your upper back, unrack it by rotating it free of the hooks, and step your feet slightly forward of where you'd stand for a free-bar squat. Because the machine handles the bar path for you, your legs can focus on depth and drive without the usual balancing act. Squat down by pushing your hips back and bending your knees, then push the floor away to stand. Keep your feet forward enough that your knees don't have to travel past your toes to hit depth on a bar that won't move with you.