You can see how "solid effect" of "stiffness" impact the boats in the video. You get super cool effects when there are a few ones, but a lot of them result in them taking a lot of time to settle. It's fine for boats as you rarely think of crowds of boats, and I just made a scene for this in the video to show that it works, and still look good for them.
But if you have a crowd of infantry constantly wiggling and taking forever to settle, this would look very bad. Hence the spring.
Inertia and pushing aren't the same thing. But for navigating on open terrain, this wouldn't change much. However if you try to funnel "solid" physical bodies into a choke-point, they obviously take a while more to achieve the same movement.
So if ever we wanted to make cavalry "solid bodies", with this PR it's only just one tweak in their template. But because big crowds of them cost more to simulate and that moving them around take longer if they are in high number, it should be accompanied by an increase of cost/population for example. The pros are that they will have much more realistic charge effects, like you see with elephants, but cavalry have less contact area and are moving much faster, the physic outcome would be different too (like a cavalry can trow fewer infantry units away but at higher velocity). So this is just a balancing change, out of scope.
Even without proper "charge", -so just using the old pushing pretty much as it was, the inertia improves physics for cavalry fighting at no additional cost. The cavalry can push away other spread units thanks to it's superior weight, and also push its target by a little - even if ideally it would be more, and the cavalry would be smart enough to take more advantage of it instead of stopping once in front of it's target.
Screencast from 2026-09-22 18-44-41.webm