The principle these people are using is called Accommodating Resistance: which is defined as the increasing of resistance through a range of motion while performing a movement. (In a majority of cases, the barbell lifts; bench, squat, deadlift.) Example: When performing a squat with chains attached to the bar, as you squat down, the chains pile up on the floor. This lightens the weight on your back until you proceed to squat up again when the chains then begin to come off the floor and progressively increase the weight on your back.
If you are having trouble with the top portion of your bench press, or the lockout portion of your deadlift, or if you want to just add some extra stimulus for leg growth, these are great tools to use.
There are several implements to use for accommodating resistance and each have their merits. However, I will rate them based on ease of use, applicability and calculation of weight used.
1. Chains. Chains are easy to use and easy to calculate. They weigh a certain amount and you can measure how much you are adding to your barbell very simply. Setting up is easy. Use some sort of lead, be it with a chain or straps (demonstrated below). Attach the chains to the lead at the proper height to where they are almost fully off the floor when you are at the top range of your motion and they should completely or nearly completely deload onto the floor when you are at the bottom end of your motion. They can also be used for deadlifts, simply by looping them over the barbell or attaching them via carabiner (It's exceptionally good for working the lock out portion).
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The picture on the right demonstrates a misguided use of chains. It's not necessarily wrong, but the amount of weight deloading isn't significant. Only a few links will deload to the floor.
2. Bands. Bands aren't nearly as easy to set up as chains, nor are they as simple to calculate the weight being added. However, they do provide an interesting stimulus and using them can cause tremendous power output. Setting up bands can be tricky. You can use a dumbbell to loop the bands around on the floor and then attach them directly to the barbell. Or, if your rack or bench has band loops, use those! Most sites like elitefts.net provide a band calculation chart that shows how much tension the band is creating at what length they are being stretched. For deadlifts, I find simply looping the band over the barbell and placing both ends under my feet to be a really simple and effective way to use them (also good for lockouts).
One can also use the bands to do a reverse banded lift. Which means the bands are suspended from above and looped around the barbell. They then stretch when you are in the bottom position of your movement and deload some of the weight, which then reloads as you move toward the top end.
3. Single Ply Squat Suits and Bench Devices like the Slingshot
Squat suits can be beneficial in keeping everything around your hips compressed. In reality a squat suit should allow you to squat a little extra weight by accommodating some of the resistance with the material it is made of, especially when down in the hole. The slingshot is a nifty little device that you loop onto your arms for bench pressing (or most pressing movments, like pushups or dumbbell pressing). The material stretches out when you bring the weight to your chest, deloading some of the resistance. As you raise the weight, the material has less stretch and the weight progressively reloads.
Accommodating resistance is a great tool to use to add a little extra stimulus for strength gains and muscle growth. It should be treated as such, extra stimulus, not the end all be all to strength training. Nothing beats straight weight on a barbell. And you should be well experienced in the principle movements before attempting using any of these implements.
