flat spin
"MACHINBIRD" IMHO if you'd ever done a flat spin, you wouldn't write :"One ominous aspect of flat spins is the eyeballs out g that results from the rotation. This would rapidly disable the crew, particularly if they did not have their shoulder harness locked."
I 've been teaching aerobatics and spin recovery ( positive, inverted, and positive flat and inverted) spins (in CAT "A" aerobatic airplanes...) for over 20 years and both my eyeballs are securely in place, or so say my FAA and JAR-FCL medicals.