For Radio Frequencies this box will provide a small 'load'. Very small as the wood is rather thick so capacitance won't be very high.
It 'works' by creating a large surface area which acts as one plate of a low value capacitor and the other plate is the 'air' and maybe even the floor a little.
You can DIY one by putting some aluminium foil in a similar sized wooden box, connect it to a banana socket and fill the box with whatever you can find that adds some weight.
It will be equally (in)effective.
A bit like someone's body when walking on shoes or slippers.
Some folks that are being 'plagued' by hum/noises that magically go away when they touch a device or the RCA connector.
Your body functions as a capacitor and a chance for RF to take the path of least resistance, in that case the body and its capacitance to ground, depending on factors about 100pF to 200pF on average.
I don't think this box is designed for this function though. It's function is to part believers from their money and give them a (false) sense of 'knowing it works' which helps getting more listening enjoyment (piece of mind is a great way for the brain to get more enjoyment).
It does not work... because when you connect the box with say 50cm or 1m of wire this will act as an antenna/inductor and provides a high resistance for RF.
It would be the equivalent of connecting a small capacitor (maybe 100pF ?) to a piece of wire and connecting that capacitor to safety ground.
Let's be clear about this... safety ground is NOT a 'ground' for radio frequency signals. It is only there for safety.
Safety ground can be helpful to create or reduce ground loops by the way.
I would assume the equipment might or might not be connected to safety ground anyway.
The crystals, of course, do nothing and are just there as weight and create a nice story to sell these overpriced boxes to people who believe in it.