I have not come across these before.  but the house I am wiring at the moment is using them.   It appears to be a thin steel "top hat" section strip. You screw it to the underside of the joists (screwing through the two sides of the "brim" and then the plasterboard screws into the flat part.   It's supposed to be a sound proofing measure and this apparently obviated  the need for any actual acoustic insulation.   Anyone used these? does it really work? I am skeptica