It's a "passive radiator." Basically replace the port (a given volume of air) with a speaker cone (with no motor/magnet, or just not connected to anything). The mass of the cone, and it's resonance point, is akin to the the mass of air in a port. Tuning the system is done by adding weight to the cone (there is often a bolt you can add washers to, often on the back, but I put mine on the front for easier tweakage) vs changing the volume of the port (length and/or width). Theoretically very similar designs but they end up not sounding exactly the same (I tend to prefer passive radiators) and end up being (usually, but not always) more easily user-tunable than ported boxes.
So.... if you ever have a drive unit with a cracked magnet (e.g. shitty stamped frame late 70s PA stuff or white van speakers) whack off the motor with a hammer and you have a PR to play with (you will likely need to add weight and stiffen the cone which can be done by painting it with puzzcoat/hodge podge or just plain old watered down white/wood glue).
Back when I ran a venue, I wanted the PA to look vintage (mostly indie rock shows), so I scored a pair of those old Traynor 4x12 vertical line arrays (I already had a pair of Sun horns to go on top) for almost nothing because half the drivers were blown. I converted the broken drivers into PRs. Later I replaced the remaining drivers with modern (heavier duty) bass drivers (an EV and a Black Widow each IIRC) which worked great because those cabs are a little on the small side for 4 modern 12s, but just 2 was about right. Those things kicked and I only ever ran them at half the power they could take.
