This is an excerpt from Powerlifting-2nd Edition by Dan Austin & James Bryan Mann.
Chains weigh from 18 to 50 pounds (8.2-22.7 kg) each. They are connected to the bar by smaller feeder chains so that when the lifter is in the bottom portion of the lift, the chain weight is entirely de-loaded onto the floor, and when the lifter is at the top portion of the lift, 80 to 90 percent of the chain is off the floor, adding resistance to the bar (figure 10.1). When using chains, you must make sure the weight properly loads and unloads. One device on the market is essentially a heavy chain on a collar that allows you to lift the entire weight off the floor, meaning very little is de-loaded. It does not elicit the desired response since there is no accommodation and the entire weight is on the bar. This is not to say that the chaotic swinging of the chains in the air doesn’t have an added training effect. It does have an added effect; the constant change of direction of the resistance acts as a proprioception device. However, the desired effect of accommodation will not be reached. Also, more effective means of chaos training exist, discussed in the section on suspended kettlebell training.
What are the benefits of adding chains? Isn’t this simply additional weight on the bar? Why not just throw another plate on the bar? When set up properly, chains allow you to accelerate through the movement. Christian Cantwell, 2008 USA Olympic silver medalist and three-time world champion in the shot put, relied on training with chains to learn to do just that. He thought that traditional weight training movements made him slow down at the top. Adding chains allowed him to accelerate throughout the entire range of the movement. He was able to improve strength and the RFD at the end of the movement so he could push the shot maximally all the way through the end of the movement and get the greatest distance possible. Without the use of the chains, he couldn’t have trained his body to perform as it did, and the results may have been quite different.