Hi-Fi Choice Review:
"Basically this is a large piece of MDF, but its weight of over 11kg makes it obvious there's more to it than that. Inside, deep channels have been cut out and filled with 2mm-diameter steel balls, which act very effectively to damp any kind of vibration."
"The result really is very dead, as one can easily confirm by tapping it and, as a result, it adds most beneficially to the performance of an unsuspended equipment rack."
"A very simple, but also effective way to reduce the amount of vibration reaching sensitive source components from below."