The reverse band squat provides an overload effect allowing you to get a feel for handling more weight. This is a form of accommodating resistance. Basically, the bands assist you at the bottom of the squat but "it's all you, bro" as you near the top coming back up.
Powerlifters use this method to increase power production and boost confidence with big loads, and even bodybuilders are using it for leg development. Here's John Meadows using this method with a yoke bar:
All you need is a set of bands and a power rack. Choose a band tension that'll allow you to handle around 15-20% over your max back squat at the top. If your max squat is 425, use a band tension that'll allow you to squat between 490 and 510. It doesn't have to be exact. Squats with reverse bands can be trained with a variety of reps, anywhere between 1-10 reps.
How you attach your bands to the rack will depend on how tall your rack is. If it's a short rack, use a doubled lighter band. Loop one end around the bar, then over the top of the rack and back to the bar:
If it's a taller rack, use a slightly heavier band, but one end will be choked around the top of the rack, the other will be looped around the bar:
The goal is to have the bands provide as little tension at the top as possible, but not be totally loose from the bar.
You may have also seen top powerlifters using chains, another form of accommodating resistance. Chains provide the reverse effect: the squat gets harder as you come back up. It's another great option if you have access to the equipment. More info here.