The band(s) add more resistance where you have the least mechanical advantage (at the top or lockout) and de-loads where you're weakest (off the floor).
Adding them can be tricky, especially if you don't have access to a platform or a fancy power rack with pegs. It often involves grabbing a few pairs of heavy dumbbells, wrapping the bands around each handle, and then splaying the bands across the barbell. Long division is easier. Check out the video for a simpler way.