Squatting with chains is a nice variation. Instead of just squatting with the barbell, you add chains to the sides so that as the squatter descends, more of the chains lay on the floor, decreasing the overall load. As the squatter ascends, the chains are raised from the floor, increasing the overall load. This requires greater force production at the top position of the squat- this is appropriate, as this is the position our legs are at a mechanical advantage to produce greater force. One of the great benefits of squatting with chains is the effect of instability it provides for the squatter, placing a higher demand for stability in their core and trunk. Another benefit is the feedback that the chains provide for your body in space. For example, if your chest is too far forward, you may find the chains to push you even further forward. The chains give feedback to move your body with proper form.
Here is a quick clip of my client squatting with chains: