So it's not actually 75hz all the time then? Depending on what's on the screen?

That's unfortunate.

I'm imagining a fast scrolling game with complex backgrounds where most of the pixels are changing values every frame, I assume it completely breaks down in that case.

It's 75hz when it needs to be, but if 2 frames are mostly identical, it doesn't needlessly move ink around. Effect: 75hz always as far as the user is concerned, but sometimes it uses less power than that when possible, due to very clever optimisations at the firmware level.

Or that's how I understand it anyway.

I saw that Alex Soto himself is in this comment thread, he'll know a lot more than me, I'm just spreading what little knowledge I've gathered from his blog posts and some of the discussions in the modos mastodon server.

I've probably misunderstood a lot of that too, I'm not a hardware engineer, just a lowly java dev with a strong but hobby level interest in eink.

Modos is my dream laptop, but it's currently unclear when that'll become reality.

Again, Alex Soto will know more.

For a fast scrolling game it makes more sense to use a display which is made for that.