Decoding the Design of In-Flight Seat Belts
atlasobscura.com
atlasobscura.com
Back in cattle-class, there just isn't much space for that acceleration.
In such a scenario, if the plan rapidly drops and you're sat, the 2-point belt at the waist will keep you on your seat. If you're lying flat and you have the same belt you'll probably just sandwich your head against your knees or worse - especially if you're lying on your side or back!
I never checked to see if there was any validity to my thoughts though.