Which device is specifically designed for moving loads that can vary in weight during movement?

The brake bar rack is specifically designed to handle loads that can change in weight, making it suitable for applications where the weight of the load might vary. This device features a system that allows for modifications in the friction applied to the moving load, ensuring both safety and efficiency during operation. The ability to adjust the braking force allows for more control and stability, which is crucial when dealing with varying weights.

In contrast, the other options do not provide the same functionality. A simple pulley is useful for lifting and moving loads but does not inherently adjust to varying weights. Similarly, a friction knot, while effective in certain climbing scenarios, does not have the versatility or designed capacity to manage changing load weights in a systematic way like the brake bar rack does. A climbing harness is mainly for user safety and support but does not focus on managing load weight during movement. This makes the brake bar rack the ideal choice for scenarios involving variable loads.