What are all these metal cans/pods sitting on top of electronics?


John Darko the youtube reviewer has them on all his equipment. I see them on alot of people's electronics it's a tweak of some sort I just don't know what they are or what they do. 

Thanks,
Steve
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He has stated they are just "paperweights" to keep the small stuff like streamers and DACs from moving with stiff cables. Not "mass loading" baloney.
Brass is a popular audiophile material used for damping electronics (and as footers under electronics and speakers). Brass weights with a thin cork or elastomer backing are sometimes used to reduce resonances from undamped metal chassis tops covering electronics.  Some use them on top of speakers to drive cabinet resonances to a lower frequency or to damp flimsy/thin speaker cabinets.
I’ve tried Herbies Audio Lab Stabilizers (or two stacked). Pre, DAC or ss amp, but I always end up removing them after some time. Same outcome every time; a bit more clarity and delineation but slight negative affect on soundstaging (within, not overall size) and also a slight loss of the organic flow I have... It’s a fine tuning tweak that not all systems need or can benefit from. 
The community will benefit from my most recent article for Dagogo.com just submitted a day or two ago. It will take on the subject of mass loading and several other tweaks.