this etsy link (!), this is a passive module that does an active module's work - it provides simple VCA control of a signal with another signal. How can something work - and light an LED! - without a power connection? Well, it uses the current of the signal and control voltages to drive the whole thing.
There are other Vactrol VCA's out there, but they aren't 2HP, and they are $32. I have this sitting right next to a Pittsburgh LFO2 in my rack, and it gives me access to a nice bit of voltage control without breaking the bank (or my HP allocation) to do it. Since this is a passive module, there will be plenty of voltage droop, so it isn't something that you'll want to use for precision VCA work. But for basic, run-of-the-mill voltage controlled depth or level adjustments, you can't really beat this little guy.