I made a couple of these a few years ago. This one uses Speakon sockets and is the better of the two. Inside is an ESP Project 33 PCB controlling a couple of high-current relays and a 15V power supply to run it. It has a 2mm tip LED in the front panel to indicate that power is on and that the relays are in the “speakers-connected” position.
It is for connection between the speaker outputs of a solid state power amplifier and the speakers. If the power amp has a switched mains output, it can be used to power this box so you don’t have to manually activate it. Otherwise you can leave the power amp front switch on and plug this into the same power strip and switch them both on and off from there. Since the photos were taken I changed the wiring to exploit the “loss-os-AC” detector so that muting is instantaneous at shut down. This is good as it mutes before any amplifier shut-down thump can get through to the speakers. It also mutes the speakers if amplifier fault DC is detected.