zerospinskierIt doesn't have to do with the geometry or look, it is the elastic travel of the binding. This means your boot can move around a lot on the binding before it actually pops out. When you slam onto a rail or land hard on ice, the sudden jolt won't knock your boot out. It will have some "give" and allow the binding to absorb the shock like suspension. This helps a lot with hard switch landings too. The heel piece on something like FKS absorbs the heel lift really well and prevents popping out and slamming your face into the ground.
Yes, you can prevent popping out with bindings that aren't good for freeskiing, but you will need to crank the DIN so your boot is simply glued tightly into place. It won't release because it won't move at all. But then in this case if you have a knee twisting fall, they won't release, making injury much more likely. The high elasticity allows us to leave the DIN low for our knees, but still prevent prerelease.
Is there a source for this? I'm doubtful because I did a lot of research when buying bindings last year and never heard anything about binding elasticity. I also don't see how a binding can be designed to stay engaged when pressing or buttering, but release just as easily when crashing. I'm not saying I can't be convinced if you give a good source, its just that I've never heard it before and it sounds like it might be made up.
To OP, I might suggest turning up your din a little bit. If you don't know, its the tension at which the binding will release, and is represented by a number on the see-through plate on each of your bindings. The higher the number, the greater amount of force is required to release your binding. When you get your skis mounted in a shop, they usually low-ball your din for safety and insurance reasons. The assumption is that if you are experienced enough to need a higher din, you also know enough about your equipment that you can adjust it yourself, at your own risk.
I'm 6'1 160lbs and my skis came back from the shop at din 8. I do lots of backcountry skiing where conditions are variable, I ski really fast through ungroomed terrain at the resort, and I also love doing butters (Like you). Since all of these things can cause unwanted pre-releases, I turned up my din to 9.5. I do this knowing that it puts me at a greater risk of knee injury, since the bindings are less likely to release when they are supposed to. However, I decided that the danger of having a ski come off at the top of an icy couloir or while going too fast in gnarly terrain outweigh the risk of a low speed crash.