>>2831672it's called a Jecklin disk. it roughly simulates some of the acoustic characteristics of the human head. in my opinion it's pretty much the best stereo technique for immersive ambient recordings (locations, nature)
it's not the best technique for absolutely everything, but it gives you all the benefits of using spaced omni mics without having to compromise level difference with time difference (by spacing them further or closer). it's almost binaural with this thing, but not so specifically binaural that it sounds like trash outside of headphones.
they're used in the studio a good amount, particularly for big sound sources though - like pep organs and orchestras
it enables me to mount both mics on one stand (tripod) and get a really sweet stereo image. other techniques (coincident techniques like XY and MS, and spaced pairs like AB omnis) just aren't very effective for this sorta thing