IsoSpeed first showed up in the first generation Domane. Since then it has spread to the Boone Cyclecross bike, the ProCaliber XC bike, the Madone race bike and the Checkpoint gravel bike.
Under the covers that say IsoSpeed in the photo is an axle connecting the top of the seat tube to the junction of the seat stays and the top tube. It turns the seat tube into a leaf spring allowing the seat to move backwards when the rider hits a bump while seated.
On the Madone (pictured above) it is adjustable and considerably more complicated than on any other Trek.
On the Checkpoint (pictured below) you get what you get because it is really simple.
You can see the axle end caps directly in line with the seat tube on the Checkpoint.

So how do I like it?

I’m not entirely sure I can tell it’s there! I had a lot of trepidation after reading numerous criticisms of it online, but I figured it would be OK as Trek usually gets it right. They did. I haven’t bothered to adjust the Madone because the factory setting (in the middle) feels “right”.
I was contemplating this on the way home from work today, riding the Checkpoint. I can whack a decent sized bump, and I feel it, but there is no sting to it. It mutes the rough stuff quite nicely. But I still feel the road and because of how it works there is no impact on pedalling.

So far, so good. I think it is clever and useful.