It's a spring reverb for your modular!
To be honest, it's hard to think of more to say than that. It works, and it sounds just like the spring reverb built into the Carlsbro keyboard amp I had when I was a teenager. The only point to be aware of when mounting this unit is that the spring tray itself needs to live somewhere -- it's attached to the module by a generous 60cm flying lead, but if you're housing it within your modular rack, it needs to go as far from the PSU as you can get it so that you don't get any induced hum. It may be sonically cleaner, if aesthetically more cluttered, to put it outside the rack entirely -- which has the added advantage of letting you smack it with your hand without killing all your precious valve-based modules. Once that's sorted out, though, you're in for lots of spoingy, boingy fun, for not much money. What's not to love?