Bands increase resistance (pull down) as you go through your stronger range of motion.
i.e. Top half of squats and deadlifts. In the above, I was pulling 315lbs + doubled up green bands. On the bottom it felt more like ~400lbs and at the top it felt like ~600lbs. Not sure if that's what it was but it sure felt that way.
I like to use them on as many exercises as I can - bench press, clean and pressing, even bicep curls.
You can also reverse the positioning of the bands and attach them to the top of the squat cage and the barbell (lightened method). Doing so provides less resistance at the bottom of the squat or deadlift.
I will also incorporate chains again once I get them cut to the lengths I need. It's the same idea as bands but the chains don't pull you down as bands do.
Check this out for more info:
Westside BarbellŪ"Often Imitated but never duplicated"
It is the December '04 article - "Chains and Bands"