Hi brass,
An hourglass shape has a variable spring rate so as you mentioned it has some initial softness with compression and then will become firmer much more rapidly as the firmer section of the spring kicks in.
Foam is a little bit different because foam materials absorb and distribute more of the compression forces over a wider surface area as the compression forces goes deeper while springs have very little hysteresis (the ability to absorb energy) so the force that goes out the bottom will be distributed over the same surface area as the force that is applied to the top (although of course this also depends on the type of spring and whether it has helical coils that attach the springs together).
With softer layers of foam underneath firmer layers then they will have less effect on how much the foam in the upper layers compresses and the firmer layers will “bend” into the softer layers more than “compress” into them. It would be similar to what is called a dominating layer where you have a firmer layer over a softer layer. This allows for a firmer sleeping surface with less of an “in the mattress” feeling but still provides some “give” under the mattress that can help with pressure points. This is one of the reasons that a box spring with springs is often used under an innerspring mattress and is an essential part of the design and performance of the “sleeping system” in terms of PPP. It’s the difference between what I call “sinking in” (with the foam surrounding you) and “sinking down” (where the upper layers are more bending into deeper layers that are softer and you aren’t compressing into the top layers as much).
There is more about this in post #16 here about variable spring rates and in post #2 here about the differences between the response curves of latex and innersprings and in post #2 here and the other posts it links to which talks more about dominant layers. It’s also similar to a mattress that is used on a flexible slatted or tension adjustable base (see post #2 here and post #3 here about box springs, flexible slat systems, and foundations).
All the layers and components of a mattress compress and interact together and will compress to different degrees depending on the layers and components that are both over and under them and these differences in the relative compression of each layer are part of what creates the “feel” and response of the mattress.
Phoenix