It is the density gradient centrifugation that utilises CsCl as the medium.
A gradient of densities from lowest to highest is formed and components sediment in the layer that corresponds to their densities.
Since CsCl has a density almost equivalent to that of DNA, it helps in isolating DNA from a sample containing DNA, RNA, and proteins.
Since proteins are heavier they sediment in the heaviest, bottom-most layer while RNA being lighter is present at the topmost layer with the lowest density.
This technique was discovered by Meselson and Stahl.